Cyproheptadine

What it is

A first-generation (sedating) antihistamine tablet or syrup, chemically 4-(5H-dibenzo[a,d]cyclohepten-5-ylidene)-1-methylpiperidine hydrochloride, molecular weight 350.89, usually supplied as 4 mg tablets or a 2 mg/5 mL solution. Unlike most antihistamines it also blocks serotonin receptors, and it has anticholinergic (atropine-like) activity. It is cleared mainly as a quaternary ammonium glucuronide in urine.

What the research says

It blocks histamine and serotonin at their receptors and has anticholinergic and sedative effects. For its approved allergy uses the evidence is old and it has been beaten by newer antihistamines in head-to-head trials: loratadine syrup reduced symptom scores significantly more than cyproheptadine solution in children with perennial allergic rhinitis, and in cold urticaria acrivastine was more effective and caused significantly less drowsiness. Its three big off-label uses rest on weaker footing. For appetite and weight gain, a systematic review of 46 articles found 39 reported significant weight gain, and a 375-person placebo-controlled trial found a small but statistically significant appetite improvement - while a retrospective study of 788 children with failure to thrive found no significant difference in age-adjusted weight. For serotonin syndrome it is the standard antidote, but the evidence is retrospective series and case reports, and reviewers say its efficacy requires further study. For childhood migraine prevention the evidence is uncontrolled. Its main harm is sedation and the anticholinergic burden it adds, which matters most in older adults.

Evidence grade: Limited evidence.

How it works

Drug class: First-generation sedating H1 antihistamine with 5-HT (serotonin) receptor antagonist and anticholinergic activity

Cyproheptadine competes with histamine for H1 receptors, which blunts the itch, wheal, sneezing and runny nose of an allergic reaction, and it competes with serotonin at its receptors, which is why it is used as an antidote to serotonin excess and is thought to drive its appetite-stimulating effect. It also blocks acetylcholine receptors (an atropine-like action), producing dry mouth, blurred vision, urinary hesitancy and constipation, and it crosses into the brain, producing sedation. It is metabolised to a quaternary ammonium glucuronide excreted in urine, and elimination slows in kidney impairment. (Source 1)

What it is used for

• These are the approved uses. The supporting trials are small and decades old. A placebo-controlled crossover trial in 18 people with idiopathic cold urticaria found cyproheptadine significantly reduced weal area after ice-cube challenge compared with placebo (p<0.01), but acrivastine was significantly better and cyproheptadine caused significantly more drowsiness than either acrivastine or placebo. In children with perennial allergic rhinitis, loratadine syrup reduced symptom scores significantly more than cyproheptadine. A 2014 Cochrane review of 73 antihistamine trials in chronic spontaneous urticaria does not report cyproheptadine among the agents with usable trial data and concluded no single H1-antihistamine stands out as most effective. Evidence: limited. (Source 2)
• The commonest off-label use and not an approved indication. A 2019 systematic review of 46 articles across 21 treatment populations (including 32 randomised trials) found 39 reported significant weight gain, but the studies were heterogeneous, few measured appetite objectively, and trials in HIV and cancer showed minimal to no benefit. The largest placebo-controlled trial (375 adults, South Korea) found a statistically significant but small appetite gain, with somnolence the commonest adverse event. Against this, a retrospective study of 788 prepubertal Taiwanese children with failure to thrive found no statistically significant difference in median age-adjusted weight between treated and untreated groups. Evidence: limited. (Source 3)
• Not an approved indication, but cyproheptadine is the drug reached for because it blocks serotonin receptors. The evidence is retrospective and anecdotal: in a retrospective series of 23 adults meeting Hunter criteria, every patient showed at least some response within 24 hours, with ICU patients given 12 mg loading then 2 mg every 2 hours. A 2026 review states its efficacy requires further study, and no standardised method of weaning it exists. There are no randomised trials, and there are published cases in which cyproheptadine produced no temporal improvement and the patient recovered anyway. Evidence: limited. (Source 4)
• Not an approved indication and the evidence is uncontrolled. The 2019 American Academy of Neurology and American Headache Society guideline systematically reviewed 15 Class I-III trials of preventive drugs for paediatric migraine; the agents it was able to rate were divalproex, onabotulinumtoxinA, amitriptyline, nimodipine, flunarizine, propranolol, topiramate and cinnarizine - cyproheptadine is not among them - and it concluded that most randomised trials of paediatric migraine preventives fail to beat placebo. What exists for cyproheptadine is a retrospective series of 155 Japanese children reporting 68.9% responders and a 12-patient open-label study whose own authors note it was not blinded and had no control group. Evidence: unknown. (Source 5)

Interactions

• Alcohol (drinking alcohol) (label): Alcohol adds to cyproheptadine's sedation. The label states antihistamines may have additive effects with alcohol and other central nervous system depressants. (Source 6)
• Monoamine oxidase inhibitors (label): MAO inhibitor therapy is a contraindication on the label, which states MAO inhibitors prolong and intensify the anticholinergic effects of antihistamines. (Source 7)
• Other anticholinergic medicines (tricyclic antidepressants, bladder antimuscarinics, antiparkinson drugs, other sedating antihistamines) (case reports): Cyproheptadine adds to a person's total anticholinergic load. In a prospective cohort of adults 65 and over, higher cumulative use of strong anticholinergics - with first-generation antihistamines among the commonest contributors - was associated with a dose-related increase in dementia risk. This is an association in observational data, not a demonstrated cause. (Source 8)
• Serotonergic medicines and supplements (SSRIs and SNRIs, tramadol, linezolid, St John's wort and other serotonergic herbal products) (case reports): This is the interaction cyproheptadine is used to treat rather than one it causes: serotonin syndrome arises from serotonergic drugs and supplements combined, and cyproheptadine is given as a serotonin antagonist. A paediatric guidance document lists herbal supplements explicitly among the causative agents. (Source 9)
• Herbal or "natural" weight-gain supplements (case reports): Several products sold as herbal weight gainers have been found to contain undeclared cyproheptadine at 2.65 to 8.6 mg per dose, and in some cases undeclared dexamethasone as well. Taking such a product alongside prescribed cyproheptadine doubles the dose without the person knowing. One unapproved cyproheptadine-containing product has been linked to hepatitis in a child. (Source 10)
• Sedating medicines of any kind (hypnotics, benzodiazepines, opioids) (label): The label groups these with alcohol as having additive effects, and warns that the drug diminishes mental alertness and motor coordination, which matters for driving and machinery. (Source 6)

Stopping it

• No withdrawal syndrome or taper for cyproheptadine was found in the literature searched, and the label gives no stopping instructions. What is documented is reversibility of a harm: in a child with hepatitis linked to a cyproheptadine-containing product, liver enzymes improved once the product was stopped. (Source 11)
• When cyproheptadine is given for serotonin syndrome there is no agreed way to come off it: the authors of a single case report state plainly that the literature contains no standardised weaning method. That report describes what happened to one patient after cardiac surgery and offers its own regimen only as a possibility, not as a schedule anyone has tested. (Source 12)
• For the appetite indication, what happens to weight after the drug is stopped has not been established. The 2019 systematic review's own closing point is that prospective randomised studies with objective measures are still needed to understand how it works at all. (Source 3)
• For the migraine indication, the Japanese retrospective series reports nothing about stopping or about what happens after the drug is withdrawn; what it does report is that children with comorbid neurodevelopmental disorders or orthostatic intolerance responded less well, so a trial of the drug may be abandoned for lack of effect rather than tapered. (Source 13)

What goes wrong

In that same trial, cyproheptadine caused significantly more drowsiness than both the comparator antihistamine and placebo. (Source 2)

• Randomized trial, Low certainty.
• Size: 18 patients.
• Who: people with idiopathic cold urticaria.
• How long: double-blind crossover.
• Result: cyproheptadine 4 mg three times daily caused significantly more drowsiness than acrivastine (p = 0.021) or placebo (p = 0.013); acrivastine and placebo did not differ from each other.
• Funding: not stated in the abstract.

Furthermore, cyproheptadine caused significantly more drowsiness than acrivastine (p = 0.021) or placebo (p = 0.013), which did not differ from each other.

In a prospective population cohort of older adults, higher cumulative use of strong anticholinergic medicines - a class in which first-generation antihistamines were among the commonest - was associated with a higher risk of incident dementia. (Source 8)

• Cohort study, Moderate certainty.
• Size: 3434 participants aged 65 or older with no dementia at entry.
• Who: members of an integrated health system in Seattle, followed every 2 years, with pharmacy dispensing data for 10 years of cumulative exposure.
• How long: mean follow-up 7.3 years.
• Result: 797 participants (23.2%) developed dementia; adjusted hazard ratios versus nonuse were 0.92 (95% CI 0.74-1.16) for 1-90 total standardised daily doses, 1.19 (0.94-1.51) for 91-365, 1.23 (0.94-1.62) for 366-1095 and 1.54 (1.21-1.96) above 1095 (test for trend P < .001)
• Funding: not stated in the abstract. This is an observational association and cannot establish cause; the most recent 12 months of use were excluded to reduce reverse causation.

The most common anticholinergic classes used were tricyclic antidepressants, first-generation antihistamines, and bladder antimuscarinics.

The label's list of reported antihistamine adverse reactions includes serious liver injury and blood dyscrasias alongside the common sedation. (Source 14)

• Official position, Certainty not rated.
• Size: not stated in the label.
• Who: people taking antihistamines including cyproheptadine.
• How long: not stated.
• Result: no frequencies are given; listed effects include sedation and sleepiness (often transient), dizziness, confusion, hallucinations and convulsions; cholestasis, hepatic failure and hepatitis; haemolytic anaemia, leukopenia, agranulocytosis and thrombocytopenia; urinary retention; hypotension and tachycardia.
• Funding: not applicable; manufacturer label.

Cholestasis, hepatic failure, hepatitis, hepatic function abnormality, dryness of mouth, epigastric distress, anorexia, nausea, vomiting, diarrhea, constipation, jaundice.

The label states antihistamines are more likely to cause dizziness, sedation and hypotension in older people, and that overdose in infants and young children can be fatal. (Source 6)

• Official position, Certainty not rated.
• Size: not stated.
• Who: elderly patients, and infants and young children.
• How long: not applicable.
• Result: no rates given; overdosage in infants and young children may produce hallucinations, central nervous system depression, convulsions, respiratory and cardiac arrest, and death.
• Funding: not applicable; manufacturer label.

Antihistamines are more likely to cause dizziness, sedation, and hypotension in elderly patients

A single published case report describes an 11-year-old who was found to have hepatitis after months of taking Apetamin, an unapproved imported appetite stimulant containing cyproheptadine, with liver enzymes improving after the product was stopped. (Source 11)

• Case report, Very low certainty.
• Size: 1 patient.
• Who: a previously healthy 11-year-old boy.
• How long: transaminases were elevated six months after starting the product.
• Result: elevated transaminases and positive anti-smooth muscle antibody; liver enzymes improved after stopping the product.
• Funding: not stated.

Limit of this finding: This is one case report in one child, and the link to the product rests only on the timing and on improvement after stopping it. The same report records a positive anti-smooth muscle antibody, a marker of autoimmune hepatitis, which the authors did not rule out. A reader should take this as a signal worth knowing about when an unapproved imported product is involved, and should not take it as evidence that cyproheptadine, or Apetamin, causes liver injury.

Apetamin is an appetite stimulant not approved by the United States Food and Drug Administration that contains cyproheptadine, lysine, and multiple B vitamins.

Herbal weight-gain supplements sold in pharmacies have been found to contain undeclared cyproheptadine at prescription-scale amounts, so someone can take the drug without knowing it. (Source 10)

• Survey study, Low certainty.
• Size: 9 herbal supplements marketed as natural weight gainers, purchased from local pharmacies in Iraq.
• Who: not applicable; product analysis by high-performance liquid chromatography.
• How long: not applicable.
• Result: cyproheptadine detected in 7 of 9 products at 2.65 to 8.6 mg per dosage unit; dexamethasone detected in all test solutions at 6.2 to 18.75 mg per dosage unit.
• Funding: not stated.

CYP was detected in seven of the nine products at levels of 2.65-8.6 mg per dosage unit.

What the evidence supports

In a placebo-controlled crossover trial in cold urticaria, cyproheptadine significantly reduced weal area, but a newer antihistamine was significantly more effective. (Source 2)

• Randomized trial, Low certainty.
• Size: 18 patients.
• Who: people with idiopathic cold urticaria.
• How long: double-blind crossover; duration per arm not stated in the abstract.
• Result: acrivastine and cyproheptadine both significantly reduced weal areas after ice cube challenge versus placebo (p less than 0.01); acrivastine was significantly more effective than cyproheptadine (p less than 0.01)
• Funding: not stated in the abstract; the trial tested a new compound (BW 825C) against an older one, which is a sponsor-typical design.

Acrivastine and cyproheptadine significantly (p less than 0.01) reduced weal areas following ice cube challenge when compared to placebo.

A systematic review found most studies of cyproheptadine as an appetite stimulant reported significant weight gain, but the literature is heterogeneous and rarely measured appetite objectively. (Source 3)

• Systematic review, Low certainty.
• Size: 46 articles across 21 different treatment populations, including 32 randomised controlled trials, 4 prospective cohorts, 4 retrospective cohorts, 4 case reports and 2 case series.
• Who: all age groups and populations in which cyproheptadine was used as an appetite stimulant.
• How long: varied across studies.
• Result: 39 of 46 articles reported significant weight gain in the sample studied; no pooled effect size was calculated; transient mild to moderate sedation was the commonest side effect.
• Funding: not stated in the abstract. The review notes studies were heterogeneous in methods and populations and that few provided objective measures of appetite change.

Of these, 39 demonstrated that CY resulted in significant weight gain in the sample under study.

The largest placebo-controlled trial of cyproheptadine for poor appetite reported a statistically significant difference between groups in appetite score, but the figures it published for that difference do not agree with each other. (Source 15)

• Randomized trial, Moderate certainty.
• Size: 375 adults randomised (189 cyproheptadine, 186 placebo)
• Who: adults aged 19 to 64 with poor appetite in South Korea.
• How long: 8 weeks.
• Result: mean (SD) change in Edmonton Symptom Assessment System appetite score -2.42 (0.12) with cyproheptadine versus -2.03 (0.13) with placebo in 375 randomised patients (189 versus 186); the paper reports the between-group difference as +0.38 (0.18), 95% CI -0.73 to -0.04, P = 0.0307, figures that are mutually inconsistent (see caveat); significant increases in weight and body mass index; somnolence the commonest adverse event; one serious adverse event (colitis) judged unlikely to be related.
• Funding: not stated in the abstract. Note that the reported point estimate (+0.38) and its stated confidence interval (-0.73 to -0.04) are not internally consistent as printed, which a reviewer should check against the full paper.

Limit of this finding: The published abstract reports the between-group difference as '+0.38 [0.18]; 95% CI, -0.73 to -0.04; P = 0.0307'. A positive point estimate cannot sit inside a confidence interval that is entirely negative, so at least one of those numbers is wrong as printed. Both groups' appetite scores also moved in the negative direction (-2.42 with cyproheptadine, -2.03 with placebo), and on this scale a lower score means better appetite, so the sign of the reported difference does not match the group means either. A reader should take from this that the trial found a small difference favouring cyproheptadine, and should not take the figure 0.38 as the size or the direction of that difference, or treat the confidence interval as usable, until the paper's full tables are checked.

The cyproheptadine group experienced a mean (SD) change in appetite score of -2.42 (0.12) compared with -2.03 (0.13) in the placebo arm, representing a statistically significant appetite gain in the cyproheptadine group (difference, +0.38 [0.18]; 95% CI, -0.73 to -0.04; P = 0.0307).

In a retrospective series of adults with serotonin syndrome, every patient showed at least some response to cyproheptadine within 24 hours. (Source 4)

• Case series, Very low certainty.
• Size: 23 adults meeting Hunter criteria.
• Who: adults admitted to a neurology department with serotonin syndrome; mean age 35.2 years, 52% female, 10 managed in intensive care.
• How long: at least 24 hours of treatment.
• Result: in a cohort of 23 patients (10 in intensive care, 13 on the ward): hyperreflexia in 100% (23 of 23), clonus 91%, tachycardia 83%, tremor 83%; all intensive care patients received a 12 mg loading dose then 2 mg every 2 hours for at least 24 hours, ward patients 4 mg three times daily; every patient showed at least some response within 24 hours; total doses and treatment length varied. With n=23, each percentage point is roughly a twentieth of one patient, so these figures are single-digit counts.
• Funding: not stated in the abstract. There is no control group, so spontaneous recovery cannot be separated from drug effect.

Every patient showed at least some response to cyproheptadine within 24 h.

A propensity-matched electronic health record study in adults found no higher risk of dizziness, sedation or hypotension with cyproheptadine-based appetite stimulants than with megestrol, and no higher risk than with other antihistamines. (Source 16)

• Cohort study, Low certainty.
• Size: patients prescribed cyproheptadine-based appetite stimulants, megestrol or antihistamines at one Korean tertiary hospital between 2004 and 2022, after propensity score matching (number not stated in the abstract)
• Who: adult patients, with subgroup analyses by age and duration of use.
• How long: outcomes assessed within 30 days of drug administration.
• Result: versus megestrol, adjusted hazard ratios 1.02 (95% CI 0.70-1.50) for dizziness, 0.53 (0.19-1.54) for sedation, 0.70 (0.34-1.44) for hypotension; versus antihistamines, 0.56 (0.41-0.78), 1.05 (0.46-2.38) and 0.65 (0.36-1.17)
• Funding: not stated in the abstract. The comparators are themselves sedating or risky drugs, so 'no difference' is not the same as 'no risk'; outcomes were drawn from routine records within 30 days only.

Limit of this finding: Two things in this paper do not line up. It describes the antihistamine comparison as showing 'similar findings' and no significant differences, but the dizziness result it reports there, a hazard ratio of 0.56 with a 95% confidence interval of 0.41 to 0.78, excludes 1 and so is statistically significant - in favour of cyproheptadine, not against it. And its conclusion speaks about 'older adults' while the study it describes enrolled adults prescribed these drugs between 2004 and 2022, without defining an older-adult group. A reader should take this as reassurance about short-term dizziness, sedation and hypotension in adults generally, and not as a specific finding about older people. Both comparison drugs are themselves sedating, so 'no worse than' is not the same as 'not sedating'.

No significant differences were observed in the risk of dizziness, sedation, or hypotension when CAS was compared to megestrol, with adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs) of 1.02 (0.70-1.50) for dizziness, 0.53 (0.19-1.54) for sedation, and 0.70 (0.34-1.44) for hypotension.

What the evidence does not support

In children with perennial allergic rhinitis, loratadine syrup reduced symptoms significantly more than cyproheptadine solution. (Source 17)

• Randomized trial, Low certainty.
• Size: 60 children.
• Who: Taiwanese children aged 2 to 12 with mite-induced perennial allergic rhinitis.
• How long: 2 weeks.
• Result: significantly greater reduction in symptom scores with loratadine (p<0.001); clinical visual analogue scale change at week 2 was 125.5 with loratadine versus 18.3 with cyproheptadine, subjective 101.4 versus 7.1, all p<0.001 in favour of loratadine.
• Funding: not stated in the abstract; loratadine is the branded comparator and the trial has no placebo arm.

After 2 weeks of treatment, there was a significantly greater reduction in symptom scores in the loratadine group than in the cyproheptadine group (p<0.001).

The same systematic review found little or no benefit in people with progressive illnesses such as HIV and cancer. (Source 3)

• Systematic review, Low certainty.
• Size: subset of the 46 included articles, number not stated.
• Who: people with malignant or progressive disease states, including HIV and cancer.
• How long: varied.
• Result: minimal to no benefit of the medication reported in these populations.
• Funding: not stated in the abstract.

Studies exploring the use of CY in those with malignant/progressive disease states, such as HIV and cancer, showed minimal to no benefit of the medication.

In a retrospective hospital study of 788 prepubertal children with failure to thrive, cyproheptadine made no statistically significant difference to age-adjusted weight. (Source 18)

• Cohort study, Very low certainty.
• Size: 788 patients aged 3 to 11 years; 50 treated and 738 untreated.
• Who: prepubertal Taiwanese children newly diagnosed with failure to thrive at one medical centre, 2007 to 2016.
• How long: mean treatment 97.22 days (range 14 to 532 days)
• Result: no statistically significant difference in median age-adjusted weight between treated and untreated groups over follow-up; within the treated group, weight and BMI were positively associated with medication duration (P = 0.026, P = 0.04)
• Funding: not stated in the abstract. The treated group was 50 children against 738 untreated, with no randomisation.

Limit of this finding: The paper's own conclusion says its findings 'underscore the positive association between cyproheptadine hydrochloride treatment and weight gain', which does not follow from the result it reports: there was no statistically significant difference in age-adjusted weight between the 50 treated and 738 untreated children. The association with treatment duration was found inside the treated group only, and children who stayed on the drug longer may differ from those who did not. A reader should take the headline result, not the conclusion sentence.

No statistically significant difference in the median age-adjusted weight value was noted between the T-group and NT-group during the follow up period.

An AAN and AHS guideline systematic review of paediatric migraine prevention found that most preventive drugs fail to beat placebo; cyproheptadine was not among the agents it was able to rate. (Source 5)

• Systematic review, Moderate certainty.
• Size: 15 Class I-III studies meeting inclusion criteria, from a systematic review of literature January 2003 to August 2017.
• Who: children and adolescents with migraine.
• How long: varied by trial.
• Result: insufficient evidence for divalproex, onabotulinumtoxinA, amitriptyline, nimodipine and flunarizine; propranolol possibly better than placebo for at least 50% headache reduction; topiramate and cinnarizine probably better; the drugs named do not include cyproheptadine.
• Funding: not stated in the abstract; American Academy of Neurology guideline process.

The majority of randomized controlled trials studying the efficacy of preventive medications for pediatric migraine fail to demonstrate superiority to placebo.

Where the evidence is mixed

In six patients with cold urticaria the authors concluded that cyproheptadine works by blocking H1 receptors rather than by reducing histamine release, and inferred from that, without testing it, that any adequately dosed H1 antihistamine should work as well. (Source 19)

• Case series, Very low certainty.
• Size: 6 patients.
• Who: people with cold urticaria.
• How long: before and after cyproheptadine therapy.
• Result: patients were asymptomatic on cyproheptadine and ice cube tests reverted to normal, with only 1 of 6 showing swelling on ice-water immersion; histamine release was unchanged after therapy in 5 of 6.
• Funding: not stated.

These data further suggest that a sufficient dose of any standard antihistamine should be similarly effective and that patients who do not tolerate cyproheptadine or do not appear to respond to it should be tried on other antihistamines of the H1 type.

A Cochrane review of H1-antihistamines for chronic spontaneous urticaria found no single antihistamine stands out, and the evidence for the class rests on few and small studies. (Source 20)

• Systematic review, Low certainty.
• Size: 73 studies, 9759 participants; 34 studies provided data for 23 comparisons.
• Who: people with chronic spontaneous urticaria.
• How long: up to two weeks (short-term) or over two weeks to three months (intermediate-term)
• Result: cetirizine 10 mg gave complete suppression of urticaria versus placebo, RR 2.72 (95% CI 1.51 to 4.91); most comparisons rested on one study and were downgraded for imprecision; evidence for quality of life was insufficient.
• Funding: not stated in the abstract; Cochrane review.

No single H1-antihistamine stands out as most effective.

A small retrospective cystic fibrosis series reported a large before-and-after change in BMI z-score, but it was uncontrolled and showed no dose-response. (Source 21)

• Cohort study, Very low certainty.
• Size: 15 patients.
• Who: paediatric patients with cystic fibrosis and suboptimal nutritional status prescribed cyproheptadine for 12 months or more, 2013 to 2018.
• How long: 12 months of treatment compared with the preceding 12 months.
• Result: mean change in BMI z-score +0.91 over 12 months of treatment versus -0.52 in the previous 12 months (p = 0.0002); lung function change +2.79% versus -6.2% (p = 0.07); no dose-response relationship observed.
• Funding: not stated in the abstract. This is a before-and-after comparison in 15 children with no control group, so regression to the mean cannot be excluded.

The mean change in BMI z-score over 12 months of treatment with CH was +0.91 compared to -0.52 in the previous 12 months

Reviewers of serotonin syndrome state that cyproheptadine's efficacy requires further study. (Source 22)

• Expert review, not systematic, Very low certainty.
• Size: not applicable; narrative review with no stated search method.
• Who: people with serotonin syndrome.
• How long: not applicable.
• Result: no effect size; the review reports that management focuses on early recognition, stopping serotonergic agents and supportive care, with cyproheptadine as a targeted treatment whose efficacy requires further study.
• Funding: not stated.

Management strategies focus on early recognition, discontinuation of serotonergic agents, supportive care, and targeted treatments such as cyproheptadine, although its efficacy requires further study.

A paediatric serotonin syndrome guidance document states that clinical trials in children are nonexistent and that much of the information comes from case reports and extrapolation from adults. (Source 9)

• Expert review, not systematic, Very low certainty.
• Size: not applicable.
• Who: children with serotonin syndrome.
• How long: most patients recover within 24 to 72 hours after treatment.
• Result: no effect size; cyproheptadine is described as usable as a serotonin antagonist in moderate to severe cases.
• Funding: not stated.

Since clinical trials in children are nonexistent, much of the information comes from case reports, narrative reviews, and extrapolation from adult studies.

The paediatric migraine evidence specific to cyproheptadine is retrospective: in 155 Japanese children, 68.9% of those evaluated were classed as responders, with lower efficacy when neurodevelopmental disorders or orthostatic intolerance were present. (Source 13)

• Cohort study, Very low certainty.
• Size: 155 children (71 male, 84 female) aged 3 to 15; efficacy evaluated in 148.
• Who: paediatric migraine patients treated with cyproheptadine at a Japanese centre; 17.4% had comorbid neurodevelopmental disorders and 14.2% orthostatic intolerance.
• How long: not stated in the abstract.
• Result: 68.9% classified as responders (at least 50% reduction in headache episodes); patients with comorbid conditions showed lower efficacy; responders required a lower dose (p = 0.039)
• Funding: not stated in the abstract. There is no control group and no blinding, so placebo response and natural history are not separated.

Efficacy was evaluated in 148 patients, with 68.9% classified as responders.

The adult migraine prophylaxis evidence for cyproheptadine is a 12-patient open-label study whose own authors state it was neither blinded nor controlled. (Source 23)

• Case series, Very low certainty.
• Size: 12 patients selected from 103 migraine patients at one hospital.
• Who: adults with frequent migraine refractory to lomerizine, valproic acid and topiramate, or intolerant of them.
• How long: 3 months.
• Result: average migraine frequency over 3 months fell to 2.6 episodes per month from a pretreatment frequency of over 10 per month (p < 0.01); initial dose 4 mg at night, increased to 8 mg/day if no significant sleepiness.
• Funding: not stated in the abstract. The authors state: this study is not a double blind randomized trial, and an open study with no control group.

Limit of this finding: This was 12 patients picked out of 103 attending one hospital, with no control group, no blinding and no randomisation; the authors say so themselves in the paper's closing sentence, and Europe PMC indexes the paper under Case Reports rather than as a trial. Their phrase 'dramatically reduced in all patients' is what an uncontrolled study looks like when everyone improves, which is also what would be expected from the natural course of migraine, regression to the mean and the placebo effect. A reader should take this as a reason someone might test cyproheptadine properly, and not as evidence that it prevents migraine.

But this study is not a double blind randomized trial, and an open study with no control group.

A much larger nested case-control study found a dose-related dementia association for strong anticholinergics overall, but the drug classes that reached significance were antidepressants, antiparkinson drugs, antipsychotics, bladder antimuscarinics and antiepileptics - not antihistamines. (Source 24)

• Case-control study, Moderate certainty.
• Size: 58,769 patients with dementia and 225,574 matched controls aged 55 or older.
• Who: English general practice patients in the QResearch database.
• How long: exposure measured 1 to 11 years before diagnosis, with sensitivity windows of 3-13 and 5-20 years.
• Result: adjusted odds ratio for dementia rose from 1.06 (95% CI 1.03-1.09) at 1-90 total standardised daily doses to 1.49 (1.44-1.54) above 1095; significant class-specific increases were seen for anticholinergic antidepressants, antiparkinson drugs, antipsychotics, bladder antimuscarinics and antiepileptics; population-attributable fraction 10.3%.
• Funding: not stated in the abstract. Observational; antihistamines are not listed among the classes with a significant class-specific association.

There were significant increases in dementia risk for the anticholinergic antidepressants (adjusted OR [AOR], 1.29; 95% CI, 1.24-1.34), antiparkinson drugs (AOR, 1.52; 95% CI, 1.16-2.00), antipsychotics (AOR, 1.70; 95% CI, 1.53-1.90), bladder antimuscarinic drugs (AOR, 1.65; 95% CI, 1.56-1.75), and antiepileptic drugs (AOR, 1.39; 95% CI, 1.22-1.57) all for more than 1095 TSDDs.

Where the research disagrees

How safe cyproheptadine is in older adults, given its anticholinergic and sedating properties

• The 2015 Adult Changes in Thought cohort (JAMA Internal Medicine), prospective population cohort of 3434 adults aged 65 and over, with first-generation antihistamines among the commonest exposures: Higher cumulative anticholinergic use is associated with an increased risk for dementia. Efforts to increase awareness among health care professionals and older adults about this potential medication-related risk are important to minimize anticholinergic use over time. (Source 8)
• A 2025 Korean electronic health record study (Journal of Clinical Medicine), retrospective propensity-matched cohort of adults with 30-day outcomes of dizziness, sedation and hypotension; the comparators are themselves sedating drugs, and the paper's conclusion generalises to older adults although the design it describes does not define that group: CAS demonstrated an acceptable safety profile in older adults, with safety comparable to both megestrol and antihistamines. (Source 16)
• The 2019 QResearch nested case-control study (JAMA Internal Medicine), nested case-control study of 58,769 dementia cases and 225,574 controls; antihistamines were not among the classes with a significant class-specific association: Exposure to several types of strong anticholinergic drugs is associated with an increased risk of dementia. These findings highlight the importance of reducing exposure to anticholinergic drugs in middle-aged and older people. (Source 24)

Whether cyproheptadine reliably produces weight gain

• A 2019 systematic review in Appetite, systematic review of 46 articles, mostly uncontrolled or heterogeneous, with few objective appetite measures: CY appears to be a safe, generally well-tolerated medication that has utility in helping facilitate weight gain in patients drawn from a variety of underweight populations. (Source 3)
• A 2021 retrospective study of 788 Taiwanese children, hospital-based retrospective comparison of 50 treated with 738 untreated children with failure to thrive: No statistically significant difference in the median age-adjusted weight value was noted between the T-group and NT-group during the follow up period. (Source 18)

How much

• Reference intake: There is no reference intake for a drug. Dosing is set by the prescriber. As a position, the FDA label (DailyMed version of 6 July 2026) states the adult therapeutic range as 4 mg to 20 mg a day, with most patients needing 12 mg to 16 mg a day. (Source 25)
• Upper limit: As a position, the same label caps adults at 0.5 mg/kg/day, children aged 2 to 6 at 12 mg a day and children aged 7 to 14 at 16 mg a day, while noting an occasional adult may require as much as 32 mg a day. Safety and effectiveness below age two have not been established, and the drug is contraindicated in newborn or premature infants. (Source 25)
• Studied: The cold urticaria crossover trial gave cyproheptadine 4 mg three times daily. (Source 2)
• Studied: The Taiwanese failure-to-thrive study used cyproheptadine hydrochloride 0.3 mg/kg daily for at least 14 days. (Source 18)
• Studied: The serotonin syndrome series gave intensive care patients a 12 mg loading dose then 2 mg every 2 hours for at least 24 hours, and ward patients 4 mg three times a day. (Source 4)
• Studied: The adult open-label migraine prophylaxis study started at 4 mg before sleep, increasing to 8 mg a day in those without clinically significant sleepiness. (Source 23)

A common belief, and what the research shows

The belief: Cyproheptadine is a harmless old allergy pill, and a safe way to build up an underweight child or adult.

What the research shows: It is harmless only relative to what it is compared against. In head-to-head trials it was outperformed: loratadine reduced symptom scores significantly more in children with allergic rhinitis, and in cold urticaria "cyproheptadine caused significantly more drowsiness than acrivastine (p = 0.021) or placebo (p = 0.013)". The label's own adverse reaction list for the class includes "Cholestasis, hepatic failure, hepatitis, hepatic function abnormality". For weight gain the picture is genuinely mixed: 39 of 46 studies reported significant weight gain, but the largest retrospective comparison in children found "No statistically significant difference in the median age-adjusted weight value". And it is an anticholinergic, a class whose cumulative use in older adults carried an adjusted hazard ratio of 1.54 for dementia at the highest exposure in one cohort. A further, specific hazard: 7 of 9 herbal "weight gainer" products tested contained undeclared cyproheptadine, so people can be taking it twice over.

Questions and answers

What is it?

Cyproheptadine is an older prescription antihistamine, usually a 4 mg tablet or a syrup. What makes it unusual is that it blocks serotonin receptors as well as histamine receptors, and it also has anticholinergic (atropine-like) activity and crosses into the brain enough to cause sedation. It is sometimes known by the brand names Periactin or Peritol. (Source 1)

What does it do in the body?

It competes with histamine at H1 receptors, which reduces itching, hives, sneezing and runny nose, and it competes with serotonin at its receptors, which is why it is used against serotonin excess and is thought to drive its appetite-stimulating effect. The anticholinergic action produces dry mouth, blurred vision and constipation, and the central effect produces drowsiness. (Source 1)

Is it good or bad for you?

It is useful in some settings and a poor first choice in others. For allergy it works but newer non-sedating antihistamines beat it in head-to-head trials and cause far less drowsiness. For appetite and weight gain a systematic review found most studies reported significant weight gain, though the trials were heterogeneous and the largest retrospective comparison in children found no difference. For serotonin syndrome it is the standard antidote on weak evidence. The main downsides are sedation and the anticholinergic burden, which matters most in older adults and in anyone driving or operating machinery. (Source 3)

How do you get more of it?

This question does not apply as it would to a nutrient. It is a prescription medicine with no food source, and the amount a person takes is set by the prescriber. As a position, the label gives an adult therapeutic range of 4 mg to 20 mg a day, most needing 12 mg to 16 mg, with weight-based dosing for children. A separate hazard is relevant here: some products sold as herbal weight gainers contain undeclared cyproheptadine. (Source 25)

If it is harmful, what reduces it?

It is cleared by the body on its own: it is turned into a glucuronide conjugate that leaves in the urine, with no unchanged drug detectable in urine even on chronic dosing of 12 to 20 mg a day. Clearance slows in kidney impairment. If it is causing problems, stopping it is the remedy - no antidote or taper is described in the literature searched. (Source 1)

Why might someone be low in it or missing it?

There is no deficiency of cyproheptadine - the body does not make or need it. The equivalent question is who cannot have it, and the label's list is substantial: newborn and premature infants, nursing mothers, people on MAO inhibitors, and people with angle-closure glaucoma, stenosing peptic ulcer, symptomatic prostate enlargement, bladder neck obstruction or pyloroduodenal obstruction. Elderly and debilitated patients are also listed. (Source 26)

Which whole foods contain it or feed it?

No whole food contains cyproheptadine; it is a synthetic drug. The only dietary route is an unintended one: analyses of herbal weight-gain products on sale in pharmacies found cyproheptadine in 7 of 9 tested products at 2.65 to 8.6 mg per dose, often alongside undeclared dexamethasone. (Source 10)

What happens if you do not have it?

Nothing happens from never taking it. For allergy there are alternatives, and the authors of the six-patient cold urticaria study suggested that an adequate dose of any standard H1 antihistamine should work about as well, so people who do not tolerate or respond to cyproheptadine can be tried on others; that was their inference rather than something they tested. For the off-label uses, going without means relying on the alternatives for appetite, serotonin syndrome or migraine prevention, each of which has its own evidence base. (Source 19)

How can you test for it?

There is no clinical blood test for cyproheptadine, and none is used to guide dosing: radiolabel studies found no detectable unchanged drug in the urine of people on 12 to 20 mg a day. Analytical methods such as high-performance liquid chromatography can detect it in products, which is how undeclared cyproheptadine has been found in herbal weight-gain supplements, but that tests the product rather than the person. (Source 1)

References

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7. DailyMed (US National Library of Medicine), label of Amneal Pharmaceuticals of New York LLC. Cyproheptadine Hydrochloride Tablets, USP (4 mg) - FDA label, Rev. 10-2025-03, DailyMed version published 06 Jul 2026 - PRECAUTIONS section (General, Information for Patients, Drug Interactions, Carcinogenesis Mutagenesis Impairment of Fertility, Pregnancy). 2026. Read the source
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18. PloS one. Effects of cyproheptadine on body weight gain in children with nonorganic failure to thrive in Taiwan: A hospital-based retrospective study.. 2021. PMID 34665812, DOI 10.1371/journal.pone.0258731. Read the source
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21. Clinical nutrition ESPEN. Cyproheptadine as an appetite stimulant in children with cystic fibrosis.. 2021. PMID 33745614, DOI 10.1016/j.clnesp.2021.01.015. Read the source
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26. DailyMed (US National Library of Medicine), label of Amneal Pharmaceuticals of New York LLC. Cyproheptadine Hydrochloride Tablets, USP (4 mg) - FDA label, Rev. 10-2025-03, DailyMed version published 06 Jul 2026 - CONTRAINDICATIONS section. 2026. Read the source