Phentermine

What it is

Phentermine is a synthetic sympathomimetic amine sold as the hydrochloride salt in tablets and capsules, most commonly 37.5 mg of the hydrochloride (equivalent to 30 mg of phentermine base). It is chemically and pharmacologically related to amphetamine and is a Schedule IV controlled substance in the United States. It is approved only as a short-term addition, described on the label as a few weeks, to a weight-reduction regimen of diet, exercise and behaviour change.

What the research says

Phentermine produces modest short-term weight loss on top of diet. The largest placebo-controlled monotherapy data put it at about 3.6 kg more than placebo at 6 months (95% CI 0.6 to 6.0 kg); a 12-week Korean randomised trial of a 30 mg controlled-release form found 8.1 kg lost versus 1.7 kg on placebo, with 95.8% versus 20.8% losing at least 5% of body weight. A four-decade meta-analysis of obesity drugs noted that placebo-subtracted weight loss for single drugs never exceeded 4.0 kg and that no drug showed clear superiority, and the label itself describes the extra weight loss as a fraction of a pound a week and the total impact as clinically limited. On harms, an 8-week blinded trial with 24-hour ambulatory monitoring found phentermine 30 mg raised heart rate by 6.2 bpm while placebo fell 1 bpm, and doubled the rate of stopping for adverse events (5.8% versus 2.2%). The famous valve and pulmonary-hypertension disasters came from phentermine combined with fenfluramine, and the label says an association with phentermine alone cannot be ruled out. Whether the drug causes genuine dependence is disputed.

Evidence grade: Limited evidence.

How it works

Drug class: Sympathomimetic amine anorectic, chemically and pharmacologically related to amphetamine; Schedule IV controlled substance in the United States

Phentermine is a stimulant-type amine closely related to amphetamine. It is given to reduce appetite, but the label is explicit that this has not been proven to be how it works for obesity: other effects on the central nervous system, or on metabolism, may also be involved. (Source 1)

What it is used for

• Placebo-controlled trials consistently show more weight loss than placebo over weeks to months, but the size is modest and the duration short. The label's own clinical-studies section states the increased weight loss is only a fraction of a pound a week and that the total impact over diet alone must be considered clinically limited. Evidence: limited. (Source 2)
• A 13,972-patient electronic-health-record cohort found people who stayed on phentermine longer lost more weight (7.4% more than the up-to-3-month group at 24 months) with no significant difference in cardiovascular events or death, which were rare (0.3%). This is observational: the design cannot rule out that people who tolerate and respond to the drug are the ones who stay on it. Evidence: limited. (Source 3)
• In a 28-week randomised trial, phentermine 7.5 mg and 15 mg alone got 43.3% and 46.2% of participants to at least 5% weight loss versus 15.5% on placebo, but both were beaten by the fixed combination with topiramate. The trial was conducted by the combination's developer. Evidence: limited. (Source 4)
• The label states that safety and efficacy of phentermine combined with any other weight-loss product, including over-the-counter and herbal preparations and SSRIs, have not been established and that such combinations are not recommended. The fen-phen episode is the historical reason. Evidence: not-supported. (Source 5)

Interactions

• Monoamine oxidase inhibitors (label): Taking phentermine during or within 14 days of an MAO inhibitor can cause a hypertensive crisis, a dangerous surge in blood pressure. This combination is a contraindication, not a caution. (Source 6)
• Alcohol (label): The label states only that drinking alcohol while taking phentermine may produce an adverse drug reaction. It does not describe a mechanism, a frequency or a study, so this is a label caution rather than a quantified interaction. (Source 7)
• Insulin and oral diabetes medicines (label): Because weight loss and appetite change alter glucose control, insulin or tablet doses may need to be reduced; the label flags that requirements may be altered. (Source 8)
• Adrenergic neuron blocking blood-pressure drugs (for example guanethidine-type agents) (label): Phentermine can blunt their blood-pressure-lowering effect. (Source 9)
• Other weight-loss products, including over-the-counter and herbal supplements, and SSRIs (label): Safety and effectiveness of phentermine combined with any other weight-loss product - prescription, over-the-counter or herbal - or with serotonergic antidepressants have not been established, and such combinations are not recommended. The valve and pulmonary-hypertension harms seen historically came from phentermine combined with fenfluramine. (Source 5)
• Food (timing rather than a true interaction) (label): No food interaction is described. The label allows the tablet to be taken with or without food, and the only timing instruction relates to sleep rather than absorption. (Source 10)

Stopping it

• The label's rule for fading effect is to stop rather than escalate: when tolerance to the appetite-suppressing effect develops, the dose should not be increased and the drug should be discontinued. (Source 11)
• After prolonged high-dose use of amphetamine-related anorectics, the label describes a withdrawal picture on abrupt stopping: extreme fatigue and mental depression, with changes on the sleep EEG. (Source 12)
• Against that, a study that deliberately interrupted treatment in patients taking phentermine for up to 21 years reported no amphetamine-like withdrawal at all beyond a return of hunger. (Source 13)
• Some stopping triggers are symptom-driven rather than planned: the label instructs discontinuation and evaluation for pulmonary hypertension if new unexplained breathlessness, chest pain, fainting or leg swelling appears. (Source 14)

What goes wrong

In the same trial phentermine 30 mg raised heart rate by 6.2 bpm while placebo fell by 1 bpm. (Source 15)

• Randomized trial, Moderate certainty.
• Size: 565 adults randomised.
• Who: Adults with overweight or obesity and at least one weight-related complication.
• How long: 8 weeks, 24-hour ambulatory monitoring at baseline and week 8.
• Result: Heart rate increase of 6.2 bpm on phentermine 30 mg versus a decrease of 1 bpm on placebo.
• Funding: industry-funded (Vivus LLC)

Also at 8 weeks, participants in the phentermine 30 mg group had an increase in heart rate of 6.2 bpm compared to those in the placebo group who had a decrease in heart rate by 1 bpm.

Stopping treatment because of an adverse event was more than twice as common on phentermine 30 mg as on placebo. (Source 16)

• Randomized trial, Moderate certainty.
• Size: 565 adults randomised.
• Who: Adults with overweight or obesity in an 8-week blinded trial.
• How long: 8 weeks.
• Result: Treatment-emergent adverse events causing discontinuation in 2.2% on placebo, 5.8% on phentermine 30 mg and 9.5% on phentermine/topiramate; most were considered drug-related.
• Funding: industry-funded (Vivus LLC)

Overall, 5.8 % of participants had TEAEs causing discontinuation from study treatment (2.2 %, 5.8 %, and 9.5 % of participants in the placebo, phentermine 30 mg, and PHEN/TPM groups, respectively); the majority were considered related to study drug.

Appetite-suppressant drugs were associated with a six-fold increase in the odds of primary pulmonary hypertension, a rare and often fatal lung disease, in the international case-control study; the exposure was mainly fenfluramine derivatives rather than phentermine alone. (Source 17)

• Case-control study, Low certainty.
• Size: 95 cases and 355 matched controls, 35 centres in four countries.
• Who: Patients with primary pulmonary hypertension in France, Belgium, the UK and the Netherlands, matched to general-practice controls on sex and age.
• How long: Exposure windows up to and beyond three months.
• Result: Odds ratio 6.3 (95% CI 3.0 to 13.2) for any anorexic-drug use; 10.1 (3.4 to 29.9) for use in the preceding year; 23.1 (6.9 to 77.7) for more than three months of use.
• Funding: not stated in the abstract; an observational association, and the drugs were mainly fenfluramine derivatives.

The use of anorexic drugs (mainly derivatives of fenfluramine) was associated with an increased risk of primary pulmonary hypertension (odds ratio with any anorexic-drug use, 6.3; 95 percent confidence interval, 3.0 to 13.2). For the use of anorexic agents in the preceding year, the odds ratio was 10.1 (95 percent confidence interval, 3.4 to 29.9). When anorexic drugs were used to a total of more than three months, the odds ratio was 23.1 (95 percent confidence interval, 6.9 to 77.7).

Echocardiography found cardiac valve abnormalities in 22.7% of people who had taken appetite suppressants versus 1.3% of matched controls, with the highest odds for fenfluramine combined with phentermine. (Source 18)

• Case-control study, Low certainty.
• Size: 257 patients and 239 controls; 233 matched pairs analysed.
• Who: Obese patients who had taken dexfenfluramine alone, dexfenfluramine with phentermine, or fenfluramine with phentermine in open-label trials 1994-1997, matched to unexposed controls.
• How long: Various exposure periods.
• Result: Valve abnormality in 1.3% of controls (3 of 233) versus 22.7% of patients (53 of 233); OR 22.6 (95% CI 7.1 to 114.2, P<0.001); OR 26.3 (7.9 to 87.1) for fenfluramine plus phentermine and 12.7 (2.9 to 56.4) for dexfenfluramine alone.
• Funding: not stated in the abstract; phentermine alone was not a study arm, so these numbers do not isolate phentermine.

A total of 1.3 percent of the controls (3 of 233) and 22.7 percent of the patients (53 of 233) met the case definition for cardiac-valve abnormalities (odds ratio, 22.6; 95 percent confidence interval, 7.1 to 114.2; P<0.001). The odds ratio for such cardiac-valve abnormalities was 12.7 (95 percent confidence interval, 2.9 to 56.4) with the use of dexfenfluramine alone, 24.5 (5.9 to 102.2) with the use of dexfenfluramine and phentermine, and 26.3 (7.9 to 87.1) with the use of fenfluramine and phentermine.

The label records rare cases of primary pulmonary hypertension and of serious regurgitant valve disease in people who reportedly took phentermine alone, and states an association cannot be ruled out. (Source 14)

• Official position, Certainty not rated.
• Size: Not quantified; spontaneous reports.
• Who: People taking phentermine, including without fenfluramine.
• How long: Not stated.
• Result: No rate given; the label requires discontinuation for new unexplained dyspnoea, angina, syncope or lower-extremity oedema.
• Funding: Regulatory position (FDA-approved label)

Limit of this finding: Section 5.2 of this label prints the clause with a plain hyphen at one end and an en dash at the other ("(PPH) - a rare, frequently fatal disease of the lungs - has been reported"). That is the label's own typography; no word is missing.

The possibility of an association between PPH and the use of phentermine alone cannot be ruled out; there have been rare cases of PPH in patients who reportedly have taken phentermine alone.

The label describes psychological dependence and a withdrawal picture after prolonged high-dose use of amphetamine-related anorectics, including extreme fatigue and mental depression on abrupt cessation. (Source 12)

• Official position, Certainty not rated.
• Size: Not quantified.
• Who: People taking amphetamines and related drugs, including at doses many times those recommended.
• How long: Prolonged high-dosage administration.
• Result: No rates given; manifestations of chronic intoxication listed as severe dermatoses, marked insomnia, irritability, hyperactivity, personality changes and, severely, a psychosis often clinically indistinguishable from schizophrenia.
• Funding: Regulatory position (FDA-approved label); phentermine is a Schedule IV controlled substance.

Abrupt cessation following prolonged high dosage administration results in extreme fatigue and mental depression; changes are also noted on the sleep EEG.

Reported adverse reactions to phentermine include raised blood pressure, palpitation, tachycardia, ischaemic events and psychosis, as well as common dry mouth and insomnia. (Source 19)

• Official position, Certainty not rated.
• Size: Not quantified; no placebo comparison given in the label.
• Who: People taking phentermine.
• How long: Any.
• Result: No rates given in the label; cardiovascular entries are primary pulmonary hypertension and/or regurgitant valvular disease, palpitation, tachycardia, elevation of blood pressure and ischaemic events.
• Funding: Regulatory position (FDA-approved label)

Cardiovascular Primary pulmonary hypertension and/or regurgitant cardiac valvular disease, palpitation, tachycardia, elevation of blood pressure, ischemic events. Central Nervous System Overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache, psychosis.

What the evidence supports

A 12-week placebo-controlled randomised trial of a 30 mg controlled-release phentermine formulation found substantially more weight loss than placebo. (Source 2)

• Randomized trial, Low certainty.
• Size: 74 participants (37 phentermine, 37 placebo)
• Who: Adults with obesity and controlled diabetes, hypertension or dyslipidaemia, in Korea.
• How long: 12 weeks.
• Result: Body weight -8.1 +/- 3.9 kg versus -1.7 +/- 2.9 kg (p < 0.001); waist circumference change 7.2 versus 2.1 cm (p < 0.001); at least 5% weight loss in 95.8% versus 20.8%; at least 10% in 62.5% versus 4.7%; no significant difference in systolic or diastolic blood pressure between groups.
• Funding: not stated in the abstract; small and single-country, and the authors themselves call for longer study.

Limit of this finding: The published abstract contradicts itself on signs and denominators. Weight change is printed as a negative number (-8.1 kg) while the waist-circumference change in the same sentence about "significant reductions" is printed as positive (7.2 cm), so one of the two conventions is wrong; both are reductions. The dispersions given for waist circumference (+/- 0.5 and +/- 0.6 cm) are implausibly small for an SD and look like standard errors. And the response percentages cannot be fractions of the 37 patients randomised per arm - 95.8%, 20.8%, 62.5% and 4.7% resolve only to much smaller analysis sets, which the abstract never states. Do not read 95.8% as 95.8% of the 37 randomised.

The participants in the phentermine DCR group showed significant reductions in body weight (-8.1 ± 3.9 vs. -1.7 ± 2.9 kg, p < 0.001) and waist circumference (7.2 ± 0.5 vs. 2.1 ± 0.6 cm, p < 0.001) compared with those in the placebo group. Weight reductions of 5% or greater from the baseline (95.8 vs. 20.8%, p < 0.001) and 10% or more (62.5 vs. 4.7%, p < 0.001) were achieved in the DCR phentermine group and placebo group, respectively.

A pooled estimate across phentermine trials put the extra weight loss at about 3.6 kg more than placebo at 6 months, with a wide confidence interval. (Source 20)

• Meta-analysis, Low certainty.
• Size: Not stated for the phentermine component; the review assessed FDA-approved and off-label weight-loss drugs.
• Who: Adults in controlled trials of weight-loss medication given with diet advice.
• How long: 6 months for the phentermine estimate.
• Result: Pooled mean difference in weight loss at 6 months 3.6 kg (95% CI 0.6 to 6.0 kg) for phentermine; 3.0 kg (CI -1.6 to 11.5) for diethylpropion.
• Funding: not stated in the abstract; the review notes heterogeneity in all meta-analyses and that long-term health-outcome studies were lacking.

A recent meta-analysis of phentermine and diethylpropion reported pooled mean differences in weight loss at 6 months of 3.6 kg (CI, 0.6 to 6.0 kg) for phentermine-treated patients and 3.0 kg (CI, -1.6 to 11.5 kg) for diethylpropion-treated patients.

In a 28-week randomised trial, phentermine alone roughly tripled the proportion of people reaching 5% weight loss compared with placebo. (Source 4)

• Randomized trial, Moderate certainty.
• Size: Seven randomised arms; total not stated in the abstract.
• Who: Obese adults, all receiving lifestyle intervention counselling.
• How long: 28 weeks.
• Result: At least 5% weight loss in 15.5% on placebo, 43.3% on phentermine 7.5 mg and 46.2% on phentermine 15 mg, versus 62.1% and 66.0% on the phentermine/topiramate combinations.
• Funding: industry-funded (trial of the phentermine/topiramate extended-release product against its own components)

The percentage of subjects achieving ≥5% WL was 15.5% for placebo, 43.3% for phentermine 7.5, 46.2% for phentermine 15, 39.2% for topiramate ER 46, 48.6% for topiramate ER 92, 62.1% for PHEN/TPM ER 7.5/46, and 66.0% for PHEN/TPM ER 15/92.

In a large health-record cohort, people who stayed on phentermine longer than the labelled few weeks lost more weight, and cardiovascular events or death were rare with no significant difference between duration groups. (Source 3)

• Cohort study, Low certainty.
• Size: 13,972 adults with a first phentermine fill 2010-2015.
• Who: 84% female, 45% white, mean age 43.5 years, mean BMI 37.8 kg/m2.
• How long: Up to 3 years of follow-up; weight compared at 6, 12 and 24 months.
• Result: Continuous use for more than 12 months was associated with 7.4% more weight loss than the up-to-3-month referent at 24 months (P < 0.001); composite cardiovascular disease or death 0.3% (41 events) with no significant difference in hazard ratios between groups.
• Funding: not stated in the abstract; observational design cannot exclude that responders are the people who continue.

Limit of this finding: The "referent group" in this sentence is defined elsewhere in the paper as phentermine use of three months or less. The quoted passage does not say so, and this is an observational health-record comparison, not a randomised one.

In multivariable models, longer-term users of phentermine experienced more weight loss; patients using continuously for > 12 months lost 7.4% more than the referent group at 24 months (P < 0.001). The composite CVD or death outcome was rare (0.3%, 41 events), with no significant difference in hazard ratios between groups.

What the evidence does not support

A meta-analysis of four decades of obesity drug trials found that placebo-subtracted weight loss for single drugs never exceeded 4.0 kg and that no drug, including phentermine, showed clear superiority. (Source 21)

• Meta-analysis, Low certainty.
• Size: 108 studies.
• Who: Randomised clinical trials of medications for obesity across four decades.
• How long: Varied; mostly short.
• Result: Placebo-subtracted weight losses for single drugs versus placebo never exceeded 4.0 kg; large effect sizes (d > 0.80) were seen only for amphetamine, benzphetamine, fenfluramine and sibutramine; no drug or class showed clear superiority.
• Funding: not stated in the abstract.

The placebo-subtracted weight losses for single drugs vs placebo included in the meta-analysis never exceeded 4.0 kg. No drug, or class of drugs, demonstrated clear superiority as an obesity medication.

The regulator-approved label itself states that the extra weight loss from anorectic drugs is only a fraction of a pound a week and that the total impact over diet alone must be considered clinically limited. (Source 22)

• Official position, Certainty not rated.
• Size: Not applicable - summary of the trial programme in the label.
• Who: Adult obese subjects instructed in dietary management in relatively short-term clinical trials.
• How long: A few weeks per trial, against a natural history measured in years.
• Result: Magnitude of increased weight loss over placebo described as only a fraction of a pound a week; rate greatest in the first weeks and decreasing thereafter.
• Funding: Regulatory position (FDA-approved label, DailyMed version published 15 June 2026)

The magnitude of increased weight loss of drug-treated patients over placebo-treated patients is only a fraction of a pound a week.

In the first blinded randomised trial to measure it with 24-hour ambulatory monitoring, phentermine 30 mg did not significantly raise systolic blood pressure versus placebo over 8 weeks. (Source 15)

• Randomized trial, Moderate certainty.
• Size: 565 adults randomised; 191 to phentermine 30 mg, 184 to placebo, 190 to phentermine/topiramate.
• Who: Adults with BMI at or above 27 and at least one weight-related complication; screening BP above 140/90 excluded.
• How long: 8 weeks.
• Result: Phentermine 30 mg versus placebo systolic difference 1.5 mmHg (95% CI -0.73, 3.73; p = 0.1867), which did not meet the non-inferiority margin either.
• Funding: industry-funded (Vivus LLC; NCT05215418)

In this study, compared to placebo at 8 weeks, phentermine 30 mg/day monotherapy increased systolic blood pressure by 1.5 mmHg (95 % CI: −0.73, 3.73 mmHg; p = 0.1867).

A clinical study that deliberately interrupted long-term phentermine treatment found no abuse, no psychological dependence, no craving and no amphetamine-like withdrawal, contradicting the label's framing. (Source 23)

• Randomized trial, Low certainty.
• Size: 269 subjects: 117 treated long-term (1.1 to 21.1 years) and 152 short-term (4 to 22 days)
• Who: Obese, overweight or formerly obese patients aged 20-88 at a weight-management practice, phentermine doses 15 to 112.5 mg per day.
• How long: Treatment durations up to 21 years, with an interruption to test withdrawal.
• Result: MINI-SUD interviews negative for abuse or psychological dependence in all long-term patients; Severity of Dependence Scale and craving questionnaire scores low in all; apart from increased hunger or eating, amphetamine-like withdrawal symptoms did not occur on abrupt cessation.
• Funding: not stated in the abstract; single-practice clinical intervention trial without a placebo control, and the questionnaires were instruments modified from amphetamine and cocaine tools.

Limit of this finding: This was an uncontrolled treatment-interruption study in 269 self-selected patients with no placebo arm. A design like that cannot establish that phentermine has no addiction potential, so the authors' absolute negatives go further than their own evidence allows.

MINI-SUD interviews were negative for phentermine abuse or psychological dependence in all LTP patients. SDS examination scores were low for all LTP and ATP patients, indicating they were not psychologically dependent upon phentermine. PCQ-NOW scores were low for all LTP and ATP patients, indicating neither short-term nor long-term phentermine treatment had induced phentermine craving. Other than an increase in hunger or eating, amphetamine-like withdrawal symptoms did not occur upon abrupt phentermine cessation as measured by sequential PWQ scores.

Where the research disagrees

Whether phentermine causes dependence, craving or a withdrawal syndrome

• FDA-approved phentermine label (Schedule IV controlled substance), DailyMed 2026, position / regulatory labelling drawing on the amphetamine class: Phentermine is related chemically and pharmacologically to amphetamine (d- and d/l-amphetamine) and other related stimulant drugs that have been extensively abused. (Source 24)
• Hendricks and colleagues, treatment-interruption study in 269 patients, International Journal of Obesity 2014, single-practice clinical intervention trial with structured dependence and craving instruments, no placebo control: Phentermine abuse or psychological dependence (addiction) does not occur in patients treated with phentermine for obesity. (Source 13)

Whether phentermine raises blood pressure

• FDA-approved phentermine label, position / regulatory labelling: Use caution in patients with even mild hypertension (risk of increase in blood pressure). (5.8) (Source 8)
• Hendricks and Greenway, 300 sequential patients in a private weight-management practice, Obesity 2011, uncontrolled retrospective record review in one private practice, patients also on a low-carbohydrate ketogenic diet: These data suggest phentermine treatment for obesity does not result in increased SBP, DBP, or HR, and that weight loss assisted with phentermine treatment is associated with favorable shifts in categorical blood pressure and retardation of progression to hypertension in obese patients. (Source 25)
• Bays and colleagues, 8-week randomised double-blind ambulatory blood pressure trial, Obesity Pillars 2024 (Vivus-funded), randomised double-blind placebo-controlled trial with 24-hour ambulatory monitoring: Similarly, the increase in heart rate with PHEN/TPM as measured by 24-h ABPM were less than those observed with phentermine 30 mg (2.6 vs. 6.2 bpm) and consistent with previous assessments of in-clinic heart rate. (Source 16)

How much

• Reference intake: Dosing is set by the prescriber. As a position, the FDA-approved phentermine hydrochloride tablet label (DailyMed version published 15 June 2026) states the dose should be individualised to the lowest effective dose, usually one tablet in the morning, and that late-evening dosing should be avoided because of insomnia. (Source 10)
• Upper limit: As a position, the label for this product provides a single strength of 37.5 mg phentermine hydrochloride (equivalent to 30 mg phentermine base) and directs that the recommended dose should not be exceeded when tolerance develops; the drug should be discontinued instead. (Source 26)
• Studied: The 12-week Korean randomised trial gave phentermine diffuse-controlled release 30 mg once daily against placebo. (Source 2)
• Studied: The 28-week randomised comparison used phentermine 7.5 mg and phentermine 15 mg as monotherapy arms alongside topiramate and the fixed combination. (Source 4)
• Studied: The 8-week ambulatory blood pressure trial used phentermine 30 mg once daily in the morning for 8 weeks against placebo and against phentermine 15 mg with topiramate 92 mg. (Source 27)
• Studied: The dependence study covered patients taking 15 to 112.5 mg per day, in some cases for up to 21 years, which is far beyond the labelled few weeks. (Source 23)

A common belief, and what the research shows

The belief: The heart-valve and pulmonary-hypertension disaster of the 1990s was caused by phentermine, so phentermine itself ruins hearts.

What the research shows: The valve damage was measured in people taking fenfluramine or dexfenfluramine, usually combined with phentermine; phentermine alone was not a study arm. The echocardiography study reports its odds ratios by exposure: The odds ratio for such cardiac-valve abnormalities was 12.7 (95 percent confidence interval, 2.9 to 56.4) with the use of dexfenfluramine alone, 24.5 (5.9 to 102.2) with the use of dexfenfluramine and phentermine, and 26.3 (7.9 to 87.1) with the use of fenfluramine and phentermine. The label does not clear phentermine either, stating of valve disease: The possibility of an association between valvular heart disease and the use of phentermine alone cannot be ruled out; there have been rare cases of valvular heart disease in patients who reportedly have taken phentermine alone.

Questions and answers

What is it?

Phentermine is a prescription stimulant-type amine, chemically and pharmacologically close to amphetamine, sold as the hydrochloride salt. The common tablet contains 37.5 mg of phentermine hydrochloride, equivalent to 30 mg of phentermine base. It is a Schedule IV controlled substance in the United States. (Source 26)

What does it do in the body?

It is given to reduce appetite, and it is grouped with the drugs called anorectics. The label is unusually candid that the mechanism is not settled: it has not been established that appetite suppression is the primary action, and other central nervous system or metabolic effects may be involved. (Source 1)

Is it good or bad for you?

Short-term, in people with obesity and under supervision, it produces real but modest extra weight loss - roughly 3.6 kg more than placebo at six months in pooled data. Against that it raises heart rate by about 6 bpm, doubles the rate of stopping for side effects, is a controlled substance, and carries rare reports of pulmonary hypertension and valve disease. The label's own verdict on the size of the benefit is blunt. (Source 22)

How do you get more of it?

There is no food or supplement route to phentermine; it exists only as a prescription controlled substance, and the labelled use is a few weeks, not an ongoing supply. Trials that went longer did so off-label: the record-based cohort studied continuous use beyond 12 months, and one interruption study included patients treated for up to 21 years at 15 to 112.5 mg per day. (Source 28)

If it is harmful, what reduces it?

Where it is doing harm the documented action is to stop it, and the label names the triggers: tolerance to the appetite effect, or new unexplained breathlessness, chest pain, fainting or leg swelling. After long high-dose use, abrupt stopping is itself described as producing extreme fatigue and low mood, so the stopping pattern matters. (Source 11)

Why might someone be low in it or missing it?

This question does not apply in the usual sense, because phentermine is not something the body makes or needs. Someone may not be taking it because the labelled use is short-term only, because of hypertension, heart disease, an MAO inhibitor or pregnancy, or because tolerance developed and the label directs discontinuation rather than a higher dose. It is also restricted as a Schedule IV controlled substance. (Source 29)

Which whole foods contain it or feed it?

No whole food contains phentermine. Diet matters in a different way: every trial that showed benefit gave the drug on top of a reduced-calorie diet, exercise and behaviour change, and the label's indication is written that way rather than as a standalone treatment. The tablet itself may be taken with or without food. (Source 28)

What happens if you do not have it?

Nobody is deficient in phentermine. Without it, the comparison is diet alone: in the 12-week placebo-controlled trial the placebo group lost 1.7 kg against 8.1 kg on phentermine, and only 20.8% of placebo patients reached 5% weight loss against 95.8% on the drug. Over years, the label states the added effect of the drug over diet alone is clinically limited. (Source 2)

How can you test for it?

There is no routine blood level test used to guide phentermine treatment. What is monitored is the response and the harms: weight and waist circumference, which the trials measured, plus blood pressure and heart rate, which the ambulatory monitoring trial showed can change (heart rate up 6.2 bpm on 30 mg). Phentermine can also show up on amphetamine immunoassay urine screens, which is a cross-reactivity issue rather than a clinical test. (Source 27)

References

1. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 12.1 Mechanism of Action. 2026. Read the source
2. Diabetes, obesity & metabolism. Randomized controlled trial to investigate the effects of a newly developed formulation of phentermine diffuse-controlled release for obesity. - section: Results. 2010. PMID 20920040, DOI 10.1111/j.1463-1326.2010.01242.x. Read the source
3. Obesity (Silver Spring, Md.). Safety and Effectiveness of Longer-Term Phentermine Use: Clinical Outcomes from an Electronic Health Record Cohort. - section: Results. 2019. PMID 30900410, DOI 10.1002/oby.22430. Read the source
4. Obesity (Silver Spring, Md.). Evaluation of phentermine and topiramate versus phentermine/topiramate extended-release in obese adults. - section: Results. 2013. PMID 24136928, DOI 10.1002/oby.20584. Read the source
5. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 5.1 Coadministration with Other Drug Products for Weight Loss. 2026. Read the source
6. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 7.1 Monoamine Oxidase Inhibitors. 2026. Read the source
7. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 7.2 Alcohol. 2026. Read the source
8. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 5 WARNINGS AND PRECAUTIONS (Highlights excerpt). 2026. Read the source
9. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 7.4 Adrenergic Neuron Blocking Drugs. 2026. Read the source
10. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 2.1 Exogenous Obesity. 2026. Read the source
11. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 5.4 Development of Tolerance, Discontinuation in Case of Tolerance. 2026. Read the source
12. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 9.3 Dependence. 2026. Read the source
13. International journal of obesity (2005). Addiction potential of phentermine prescribed during long-term treatment of obesity. - section: Conclusions. 2014. PMID 23736363, DOI 10.1038/ijo.2013.74. Read the source
14. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 5.2 Primary Pulmonary Hypertension. 2026. Read the source
15. Obesity Pillars. Effects of phentermine / topiramate extended-release, phentermine, and placebo on ambulatory blood pressure monitoring in adults with overweight or obesity: A randomized, double-blind, placebo-controlled study - section: Discussion - phentermine comparator paragraph. 2024. PMID 38304225, DOI 10.1016/j.obpill.2024.100102. Read the source
16. Obesity Pillars. Effects of phentermine / topiramate extended-release, phentermine, and placebo on ambulatory blood pressure monitoring in adults with overweight or obesity: A randomized, double-blind, placebo-controlled study - section: Discussion - safety paragraph. 2024. PMID 38304225, DOI 10.1016/j.obpill.2024.100102. Read the source
17. The New England journal of medicine. Appetite-suppressant drugs and the risk of primary pulmonary hypertension. International Primary Pulmonary Hypertension Study Group. - section: Results. 1996. PMID 8692238, DOI 10.1056/nejm199608293350901. Read the source
18. The New England journal of medicine. The prevalence of cardiac valvular insufficiency assessed by transthoracic echocardiography in obese patients treated with appetite-suppressant drugs. - section: Results. 1998. PMID 9731086, DOI 10.1056/nejm199809103391101. Read the source
19. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 6 ADVERSE REACTIONS. 2026. Read the source
20. Annals of internal medicine. Meta-analysis: pharmacologic treatment of obesity. - section: Data synthesis (complete). 2005. PMID 15809465, DOI 10.7326/0003-4819-142-7-200504050-00012. Read the source
21. International journal of obesity and related metabolic disorders : journal of the International Association for the Study of Obesity. Pharmacotherapy for obesity: a quantitative analysis of four decades of published randomized clinical trials. - section: Result. 2002. PMID 11850760, DOI 10.1038/sj.ijo.0801889. Read the source
22. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 14 CLINICAL STUDIES. 2026. Read the source
23. International journal of obesity (2005). Addiction potential of phentermine prescribed during long-term treatment of obesity. - section: Results. 2014. PMID 23736363, DOI 10.1038/ijo.2013.74. Read the source
24. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 5.6 Risk of Abuse and Dependence. 2026. Read the source
25. Obesity (Silver Spring, Md.). Blood pressure and heart rate effects, weight loss and maintenance during long-term phentermine pharmacotherapy for obesity. - section: Abstract (complete). 2011. PMID 21527891, DOI 10.1038/oby.2011.94. Read the source
26. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 3 DOSAGE FORMS AND STRENGTHS. 2026. Read the source
27. Obesity Pillars. Effects of phentermine / topiramate extended-release, phentermine, and placebo on ambulatory blood pressure monitoring in adults with overweight or obesity: A randomized, double-blind, placebo-controlled study - section: Methods (structured abstract, complete). 2024. PMID 38304225, DOI 10.1016/j.obpill.2024.100102. Read the source
28. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 1 INDICATIONS AND USAGE. 2026. Read the source
29. DailyMed / US National Library of Medicine (FDA label). PHENTERMINE HYDROCHLORIDE tablet - FDA prescribing information (Structured Product Label), Aurolife Pharma LLC - section: 9.1 Controlled Substance. 2026. Read the source