Metronidazole

What it is

Metronidazole is a synthetic nitroimidazole antimicrobial taken by mouth, by vein, or applied to the skin or vagina. The oral tablets contain 250 mg or 500 mg of the drug. It works against anaerobic bacteria (organisms that grow without oxygen) and against several protozoal parasites. Its US label has carried a boxed warning about rodent carcinogenicity since the drug's early years.

What the research says

The evidence is strongest for the infections it is licensed to treat: a randomised trial in 623 women found 7 days of metronidazole cleared Trichomonas vaginalis better than a single 2 g dose, and a Cochrane review found vancomycin, not metronidazole, is the better antibiotic for Clostridioides difficile diarrhoea. The famous warning not to drink alcohol on metronidazole is a label position that controlled human experiments have not reproduced. Rare but serious nervous-system harms - encephalopathy and peripheral neuropathy - are documented in systematic reviews of case reports and of dose-related clinical studies.

Evidence grade: Well established.

How it works

Drug class: Nitroimidazole antimicrobial (antibacterial against obligate anaerobes, and antiprotozoal)

Metronidazole diffuses into bacteria and protozoa. Inside organisms that live without oxygen, it is chemically reduced into a short-lived free radical that damages their DNA, stopping DNA synthesis and killing the cell. The label itself says the precise mechanism is not fully understood. Because the activating step needs an oxygen-poor environment, the drug does not act on ordinary aerobic bacteria. (Source 1)

Boxed warning

Metronidazole has been shown to be carcinogenic in mice and rats (see PRECAUTIONS). Unnecessary use of the drug should be avoided. Its use should be reserved for conditions described in the INDICATIONS AND USAGE section below.

(Source 2)

What it is used for

• Randomised evidence supports nitroimidazole treatment, and a 2018 randomised trial of 623 women found a 7-day course left 11% still infected at test-of-cure versus 19% after a single 2 g dose. A 2017 meta-analysis of six trials found the same direction of effect. Evidence: established. (Source 3)
• This is a licensed use recorded on the FDA label as of its 2024-07-01 version. We did not locate a recent systematic review of outcome trials for this indication, so the strength of the modern trial evidence is unclear to us. Evidence: limited. (Source 4)
• A licensed use listed on the FDA label as of 2024-07-01. The label is a regulatory position; we did not find a current systematic review quantifying absolute cure rates against comparators for this group of infections. Evidence: limited. (Source 5)
• Approved for the 750 mg extended-release tablet. A 2025 meta-analysis of 7 randomised trials found oral and intravaginal metronidazole gave equivalent clinical cure (pooled RR 1.00, 95% CI 0.94-1.06), with more gastrointestinal side effects on the oral route. Evidence: established. (Source 6)
• An individual-participant-data meta-analysis of 23 trials in 11,979 participants found metronidazole gave an odds ratio for preterm delivery of 1.00 (95% CI 0.84 to 1.17) against control, and no benefit in any subgroup or when started earlier. Evidence: not-supported. (Source 7)
• Not listed as an indication on the metronidazole tablet label we read. A 2017 Cochrane review of 22 trials found vancomycin beat metronidazole for symptomatic cure (79% versus 72%, RR 0.90, 95% CI 0.84 to 0.97, moderate quality evidence), with metronidazole far cheaper. Evidence: disputed. (Source 8)
• The label states plainly that Crohn's disease is not an approved indication, and records reports of breast and colon cancer in Crohn's patients given high-dose, long-course metronidazole without a causal relationship being established. Evidence: unknown. (Source 9)

Interactions

• Alcohol (ethanol) and products containing propylene glycol (label): The label says drinking alcohol during and for at least three days after metronidazole can cause cramps, nausea, vomiting, headache and flushing, and makes alcohol a contraindication. Controlled human experiments and a matched case-control study have not reproduced this, and a 2026 systematic review found the evidence does not strongly support it. Treat the label as a regulatory position of its date, not as the trial evidence. (Source 10)
• Alcohol (ethanol) - what the controlled data show (clinical trial): In a double-blind study of 12 healthy men, five days of metronidazole did not raise blood acetaldehyde after a 0.4 g/kg dose of ethanol and produced no objective or subjective disulfiram-like signs. Limit: This was one study in 12 healthy men, only six of whom took metronidazole. It can show that nothing happened in those six; it cannot show that the reaction never happens. The authors say as much in the sentence that ends the paper: a disulfiram-like reaction may still occur in some groups of people, or by a route other than the enzyme they tested. (Source 11)
• Warfarin and other oral coumarin anticoagulants (label): Metronidazole has been reported to increase the blood-thinning effect of warfarin and lengthen the prothrombin time, so clotting tests are monitored when the two are used together. (Source 12)
• Lithium (case reports): In people on relatively high lithium doses, a short course of metronidazole has been associated with a rise in serum lithium and, in a few cases, signs of lithium toxicity. (Source 12)
• Disulfiram (case reports): Psychotic reactions have been reported when metronidazole and disulfiram were used together; the label says metronidazole should not be given to anyone who has taken disulfiram in the previous two weeks. (Source 12)
• Busulfan (label): Metronidazole raises blood levels of busulfan, a chemotherapy drug, which can increase the risk of serious busulfan toxicity. (Source 12)
• Phenytoin, phenobarbital and other liver enzyme inducers (label): Drugs that speed up liver enzymes can clear metronidazole faster and lower its blood levels; metronidazole in turn can impair the clearance of phenytoin. (Source 12)
• Cimetidine and other liver enzyme inhibitors (label): Drugs that slow liver enzyme activity, such as cimetidine, can lengthen metronidazole's half-life and reduce how quickly the body clears it. (Source 12)
• Drugs that prolong the QT interval (label): QT prolongation on the electrocardiogram has been reported, particularly when metronidazole is combined with other drugs that can lengthen the QT interval. (Source 12)

Stopping it

• Metronidazole is a short antimicrobial course, not a long-term medicine, and the literature describes no dependence or withdrawal syndrome. What the literature does describe is recovery after stopping: in a systematic review of 136 published cases of metronidazole-induced encephalopathy, most patients improved significantly once the drug was discontinued. (Source 13)
• Peripheral neuropathy from metronidazole also tends to resolve after the drug is stopped, according to a systematic review of case reports and clinical studies. (Source 14)
• The label's own stopping instruction concerns alcohol rather than the drug: it says to avoid alcohol and propylene glycol during therapy and for at least three days after it ends. This is a label position of its 2024-07-01 date and the controlled human data do not support the underlying reaction. (Source 10)
• The label also directs prompt reassessment of whether to continue when neurological signs appear, rather than a taper. (Source 15)

What goes wrong

Metronidazole-induced encephalopathy is documented in a systematic review of 136 published patients and is usually reversible after stopping the drug. (Source 13)

• Systematic review, Low certainty.
• Size: 112 papers comprising 136 patients, from 779 publications screened.
• Who: Patients developing central nervous system symptoms during metronidazole treatment; liver disease was the most common pre-existing condition.
• How long: Varies; onset in relation to newly initiated or prolonged treatment.
• Result: Characteristic reversible symmetrical hyperintense T2/FLAIR lesions of the dentate nuclei on MRI in 90% of patients; most patients improved significantly after discontinuation. No denominator of exposed patients exists, so an incidence cannot be calculated from this design.
• Funding: not stated in the abstract.

MRI showed a characteristic pattern of reversible symmetrical hyperintense lesions on T2/FLAIR of the dentate nuclei in 90% of patients. Most patients improved significantly after discontinuation of metronidazole.

Peripheral neuropathy on metronidazole was far more common in clinical studies when the cumulative dose exceeded 42 g (about four weeks of treatment). (Source 14)

• Systematic review, Low certainty.
• Size: 36 case reports covering 40 unique patients, plus 13 clinical studies.
• Who: Patients receiving metronidazole; 31 of 40 case patients had received more than 42 g total (more than 4 weeks)
• How long: Courses up to and beyond 4 weeks.
• Result: Rates of peripheral neuropathy across the 13 clinical studies ranged from 0 to 50%; incidence was 17.9% with more than 42 g total versus 1.7% with 42 g or less. The review states the overall incidence is unknown.
• Funding: not stated in the abstract.

Within these clinical studies, we found a higher incidence of peripheral neuropathy in patients receiving >42 g total (>4 weeks) of metronidazole compared with those patients receiving ≤42 g total (17.9% vs. 1.7%).

Metronidazole caused tumours in rats and mice in several lifetime animal studies; this is the basis of the boxed warning, and it is animal, not human, evidence. (Source 16)

• Animal study, Certainty not rated.
• Size: Six reported mouse studies plus rat studies; two lifetime hamster studies.
• Who: Mice, rats and hamsters given oral metronidazole.
• How long: Lifetime feeding in several studies.
• Result: Pulmonary tumours in all six mouse studies; malignant liver tumours increased in male mice at approximately 1,500 mg/m2 (similar to the maximum recommended daily dose on a body-surface-area basis); mammary and hepatic tumours increased in female rats. Two hamster studies were negative.
• Funding: FDA-approved labelling; studies not individually attributed.

Pulmonary tumors have been observed in all six reported studies in the mouse, including one study in which the animals were dosed on an intermittent schedule (administration during every fourth week only). Malignant liver tumors were increased in male mice treated at approximately 1,500 mg/m2 (similar to the maximum recommended daily dose, based on body surface area comparisons). Malignant lymphomas and pulmonary neoplasms were also increased with lifetime feeding of the drug to mice. Mammary and hepatic tumors were increased among female rats administered oral metronidazole compared to concurrent controls. Two lifetime tumorigenicity studies in hamsters have been performed and reported to be negative.

Severe cutaneous reactions, seizures, aseptic meningitis and optic neuropathy are listed label harms without frequencies. (Source 15)

• Official position, Certainty not rated.
• Size: not stated.
• Who: Patients taking metronidazole, from spontaneous reports and literature.
• How long: not stated.
• Result: No rates are given in the WARNINGS section; the label states CNS symptoms are generally reversible within days to weeks on stopping the drug.
• Funding: FDA-approved labelling.

Severe cutaneous adverse reactions (SCARs) including toxic epidermal necrolysis (TEN), Stevens-Johnson syndrome (SJS), drug reaction with eosinophilia and systemic symptoms (DRESS), and acute generalized exanthematous pustulosis (AGEP) have been reported with the use of metronidazole.

The FDA label states as a position, not as trial evidence, that oral metronidazole is associated with a disulfiram-like reaction to alcohol. (Source 10)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: Patients prescribed oral metronidazole.
• How long: Label advises avoiding alcohol during and for at least three days after therapy.
• Result: No frequency, denominator or study is cited in the label for this statement. This is the regulator-approved position as of the 2024-07-01 version of this label, and it conflicts with the controlled human data above.
• Funding: FDA-approved labelling.

Use of oral metronidazole is associated with a disulfiram-like reaction to alcohol, including abdominal cramps, nausea, vomiting, headaches, and flushing.

In the same meta-analysis, gastrointestinal side effects were significantly more frequent with oral than with intravaginal metronidazole; this was the review's only statistically significant result. (Source 17)

• Meta-analysis, Moderate certainty.
• Size: Seven RCTs, including 697 patients.
• Who: Women treated for bacterial vaginosis.
• How long: not stated in the abstract.
• Result: RR = 2.29; 95 % CI 1.57-3.35; p < 0.001, oral versus intravaginal.
• Funding: not stated.

Gastrointestinal side effects were significantly more frequent in the oral MNZ group than in the intravaginal MNZ group (RR = 2.29; 95 % CI 1.57-3.35; p < 0.001).

What the evidence supports

A randomised trial found a 7-day metronidazole course cleared Trichomonas vaginalis in more women than a single 2 g dose. (Source 18)

• Randomized trial, Moderate certainty.
• Size: 623 women (311 single-dose, 312 seven-day-dose), intention to treat.
• Who: HIV-uninfected, non-pregnant women positive for T. vaginalis at three US sexual health clinics.
• How long: Treatment 1 day or 7 days; test-of-cure 4 weeks after completing treatment.
• Result: 34 (11%) of 312 in the 7-day group versus 58 (19%) of 311 in the single-dose group were T. vaginalis positive at test-of-cure; relative risk 0.55 (95% CI 0.34-0.70; p<0.0001). The trial stopped early for funding reasons short of its planned 1664 participants.
• Funding: National Institutes of Health.

Limit of this finding: The trial's own counts and its printed statistics do not agree. It reports 34 of 312 women still infected after the 7-day course against 58 of 311 after the single dose, and prints a relative risk of 0.55 with a 95% confidence interval of 0.34 to 0.70 and p below 0.0001. Those counts actually give a risk ratio of about 0.59, a wider interval of roughly 0.39 to 0.88, and a p-value nearer 0.01, and the printed interval is not centred on its own point estimate. Take the direction of the result - fewer infections after seven days - as the finding, and do not rely on the printed precision or the p-value.

Patients in the 7-day-dose group were less likely to be T vaginalis positive at test-of-cure than those in the single-dose group (34 [11%] of 312 vs 58 [19%] of 311, relative risk 0·55, 95% CI 0·34-0·70; p<0·0001)

A meta-analysis of six trials found single-dose metronidazole failed more often than multidose for trichomoniasis in women. (Source 19)

• Meta-analysis, Low certainty.
• Size: 6 trials met eligibility from 487 articles screened.
• Who: Women treated for Trichomonas vaginalis with oral metronidazole.
• How long: Varied by trial; treatment failure as reported.
• Result: Pooled risk ratio for treatment failure with single dose versus multidose 1.87 (95% CI 1.23-2.82; P < 0.01); 1.80 (95% CI 1.07-3.02) excluding the one study with HIV-positive women.
• Funding: not stated.

The pooled risk ratio indicated higher treatment failure for single dose compared to multidose 1.87 (95% confidence interval, 1.23-2.82; P < 0.01)

What the evidence does not support

A Cochrane review found vancomycin more effective than metronidazole for symptomatic cure of C. difficile diarrhoea. (Source 8)

• Systematic review, Moderate certainty.
• Size: 22 studies, 3215 participants overall; 444 metronidazole and 428 vancomycin patients in this comparison.
• Who: Adults with mostly mild to moderate C. difficile infection able to tolerate oral antibiotics; most trials excluded severe disease.
• How long: Treatment courses as given in the included trials.
• Result: Symptomatic cure 72% (318/444) on metronidazole versus 79% (339/428) on vancomycin; RR 0.90, 95% CI 0.84 to 0.97, moderate quality evidence. The review rated risk of bias high for 17 of 22 included studies.
• Funding: not stated in the abstract.

Seventy-two per cent (318/444) of metronidazole patients achieved symptomatic cure compared to 79% (339/428) of vancomycin patients (RR 0.90, 95% CI 0.84 to 0.97; moderate quality evidence).

An individual-participant-data meta-analysis found metronidazole for bacterial vaginosis in pregnancy did not reduce preterm delivery. (Source 7)

• Meta-analysis, Moderate certainty.
• Size: 23 eligible trials, 11,979 participants; 13 studies (6915 participants) provided individual participant data.
• Who: Asymptomatic pregnant individuals with bacterial vaginosis randomised to antibiotics or control.
• How long: Pregnancy to delivery.
• Result: Odds ratio for preterm delivery with metronidazole versus placebo 1.00 (95% CI 0.84, 1.17), I2 = 62% before imputation and 0.95 (95% CI 0.81, 1.11) after imputation of missing-IPD studies. Time-to-delivery did not differ from null.
• Funding: not stated in the abstract.

Odds ratios for PTD for metronidazole and clindamycin versus placebo were 1.00 (95% CI 0.84, 1.17), I2 = 62%, and 0.59 (95% CI 0.42, 0.82), I2 = 0 before; and 0.95 (95% CI 0.81, 1.11), I2 = 59%, and 0.90 (95% CI: 0.72, 1.12), I2 = 0, after imputation.

A controlled human experiment found metronidazole did not raise blood acetaldehyde or produce disulfiram-like effects with alcohol. (Source 11)

• Randomized trial, Low certainty.
• Size: 12 healthy male volunteers (6 metronidazole, 6 placebo)
• Who: Healthy male volunteers given ethanol 0.4 g/kg.
• How long: Metronidazole or placebo for 5 days; sampling every 20 minutes for 4 hours.
• Result: Metronidazole did not raise blood acetaldehyde and produced no objective or subjective adverse effects with ethanol. The authors note a reaction might still occur in some subgroups or by other mechanisms.
• Funding: not stated in the abstract.

Limit of this finding: This was one study in 12 healthy men, only six of whom took metronidazole. It can show that nothing happened in those six; it cannot show that the reaction never happens. The authors say as much in the sentence that ends the paper: a disulfiram-like reaction may still occur in some groups of people, or by a route other than the enzyme they tested.

Metronidazole did not raise blood acetaldehyde or have any objective or subjective adverse effects when used together with ethanol.

A review of the published case reports behind the metronidazole-alcohol warning found none of them provided evidence that could justify the conclusion. (Source 20)

• Expert review, not systematic, Very low certainty.
• Size: Six case reports involving eight patients, from a MEDLINE search 1964 to June 1999.
• Who: Patients described in reports published between 1969 and 1982.
• How long: not applicable.
• Result: Four of the eight cases were serious, including one death, but the review found no convincing evidence the reaction exists and says none of the reports provided justifying evidence.
• Funding: not stated in the abstract.

Limit of this finding: This review found the published case reports unconvincing, not harmless. Four of the eight patients described were seriously ill and one died. What the authors said was missing was evidence that metronidazole plus alcohol caused those events - not evidence that nothing had happened. Absence of convincing proof is not proof of absence.

However, review of reports published between 1969 and 1982 produced no convincing evidence that this reaction exists.

A matched case-control chart review found no disulfiram-like reactions in patients given metronidazole while alcohol was present in their blood. (Source 21)

• Case-control study, Very low certainty.
• Size: 36 patients: 18 given metronidazole with detectable ethanol and 18 matched controls.
• Who: Emergency department patients with a detectable ethanol concentration; mean age 46, mean ethanol 0.21 g/dL.
• How long: Chart review December 2010 to December 2020.
• Result: No patient who received metronidazole with a detectable ethanol concentration had a suspected disulfiram-like reaction documented. Hypertension was less frequent in the metronidazole group (16.7% vs 61.1%).
• Funding: not stated in the abstract.

Limit of this finding: Two problems sit in the source itself. First, the one statistically significant result it reports - less high blood pressure in the metronidazole group, 16.7% against 61.1% - is printed as a p-value below 0.0001, which 18 patients per group cannot produce; those percentages are 3 of 18 against 11 of 18, which gives a p-value of about 0.015. Second, the national library's copy of this abstract carries a text error, "P $lt;", where the less-than sign belongs, and the passage is quoted here exactly as that record prints it. The study is also a look back through the notes of 36 patients, far too small to show that a rare reaction does not exist.

No patients who received metronidazole and had a detectable ethanol concentration had a suspected disulfiram-like reaction documented in the medical record.

A 2026 systematic review concluded the evidence does not strongly support a clinically significant metronidazole-alcohol interaction. (Source 22)

• Systematic review, Low certainty.
• Size: 11 studies met inclusion criteria: 4 case reports, 4 clinical trials, 1 cross-sectional chart review, 2 animal studies.
• Who: Humans and animals exposed to oral 5-nitroimidazoles and alcohol.
• How long: Literature from 1 January 1970 to 22 November 2024.
• Result: All 4 case reports and 1 older clinical trial reported a potential link; 3 clinical trials, 1 cross-sectional chart review and 2 experimental animal studies did not.
• Funding: not stated in the abstract.

Thus, the available evidence does not strongly support a clinically significant interaction between alcohol and oral metronidazole, calling into question the need for strict alcohol abstinence during treatment.

Where the evidence is mixed

Oral and intravaginal metronidazole cured bacterial vaginosis equally, but the oral route caused more gastrointestinal side effects. (Source 17)

• Meta-analysis, Low certainty.
• Size: 7 randomised controlled trials, 697 patients.
• Who: Patients treated for bacterial vaginosis.
• How long: Trials published to March 2025; durations as per included RCTs.
• Result: Clinical cure pooled RR 1.00 (95% CI 0.94-1.06; p = 0.90); microbiological cure RR 0.95 (95% CI 0.75-1.71; p = 0.66); gastrointestinal adverse events RR 2.29 (95% CI 1.57-3.35; p < 0.001) higher with oral metronidazole.
• Funding: not stated in the abstract.

Limit of this finding: One figure in this review cannot be right as printed. For microbiological cure it gives a risk ratio of 0.95 with a 95% confidence interval of 0.75 to 1.71, but an interval running from 0.75 to 1.71 sits around 1.13, not 0.95, so the two cannot belong together and the upper bound is most likely a misprint. The clinical cure figure quoted here, risk ratio 1.00 with a 95% confidence interval of 0.94 to 1.06, is internally consistent. Do not read the microbiological cure number as a measurement of anything.

Clinical cure rates were equivalent between oral and intravaginal MNZ groups (pooled RR, 1.00; 95 % CI 0.94-1.06; p = 0.90).

In the same review of the metronidazole-alcohol case reports, four of the eight patients described were seriously ill and one died, although the review judged that none of the reports showed metronidazole and alcohol to be the cause. (Source 20)

• Expert review, not systematic, Very low certainty.
• Size: Six case reports involving eight patients.
• Who: People described in reports published between 1969 and 1982 as reacting to metronidazole with alcohol.
• How long: not applicable.
• Result: Four of eight cases serious, including one death.
• Funding: not stated.

Limit of this finding: This review found the published case reports unconvincing, not harmless. Four of the eight patients described were seriously ill and one died. What the authors said was missing was evidence that metronidazole plus alcohol caused those events - not evidence that nothing had happened. Absence of convincing proof is not proof of absence.

Four of the eight cases were serious, including one death, but the authors of all the reports presumed the metronidazole-ethanol reaction to be an established pharmacologic fact. None provided evidence that could justify their conclusions.

Where the research disagrees

Whether metronidazole genuinely causes a disulfiram-like reaction with alcohol

• The FDA-approved US label for metronidazole tablets, version dated 2024-07-01, Regulatory position; no study, frequency or denominator cited in the section: Use of oral metronidazole is associated with a disulfiram-like reaction to alcohol (Source 10)
• Visapaa and colleagues, Annals of Pharmacotherapy 2002, Double-blind study in 12 healthy male volunteers with repeated acetaldehyde measurement: Metronidazole did not raise blood acetaldehyde or have any objective (Source 11)
• A 2026 systematic review in Sexually Transmitted Diseases, Systematic review of 11 studies from 1970 to November 2024: the available evidence does not strongly support a clinically significant interaction between alcohol and oral metronidazole (Source 22)

How much

• Reference intake: Dosing is set by the prescriber, not by the reader. The FDA label records, as a position in its 2024-07-01 version, 250 mg three times daily for seven days or 2 g as a single or divided one-day dose for trichomoniasis in women, 750 mg three times daily for 5 to 10 days for acute intestinal amebiasis, and 7.5 mg/kg every six hours (about 500 mg for a 70 kg adult) for serious anaerobic infections. (Source 23)
• Upper limit: The label's stated maximum for anaerobic infections is 4 g in 24 hours, and it directs a 50% dose reduction in severe hepatic impairment (Child-Pugh C). This is a label position, not a tolerable upper intake level. (Source 23)
• Studied: The 2018 randomised trial gave either a single 2 g dose or 500 mg twice daily for 7 days for trichomoniasis. (Source 18)
• Studied: The label's bacterial vaginosis regimen for the 750 mg extended-release tablet is 750 mg once daily for seven consecutive days, taken fasting. (Source 24)

A common belief, and what the research shows

The belief: You must never touch alcohol on metronidazole or you will have a violent disulfiram reaction.

What the research shows: The warning is on the label and the label is a regulatory position, but the primary literature behind it is weak. A review of the original case reports concluded that none of them justified their own conclusion: "None provided evidence that could justify their conclusions." A double-blind volunteer study found metronidazole "did not raise blood acetaldehyde or have any objective or subjective adverse effects when used together with ethanol", and a 2026 systematic review found "the available evidence does not strongly support a clinically significant interaction between alcohol and oral metronidazole". None of this is a recommendation to drink; it is a statement about what the studies show.

Questions and answers

What is it?

Metronidazole is a synthetic nitroimidazole antimicrobial. It is sold as 250 mg and 500 mg tablets for swallowing, and also as an injection, a skin gel and vaginal preparations. It acts against bacteria that grow without oxygen and against several protozoal parasites such as Trichomonas vaginalis. (Source 25)

What does it do in the body?

Metronidazole gets into bacteria and parasites by simple diffusion. Inside organisms living without oxygen it is chemically reduced into a short-lived free radical that damages their DNA, which stops DNA being made and kills the cell. The label itself admits the precise mechanism is not fully worked out. Because the activating step needs an oxygen-poor environment, it has no effect on ordinary oxygen-using bacteria. (Source 1)

Is it good or bad for you?

It depends entirely on the situation. For a confirmed Trichomonas infection the randomised evidence is good: a 7-day course left 11% of 312 women still infected versus 19% of 311 given a single 2 g dose. For Clostridioides difficile diarrhoea a Cochrane review found vancomycin did better. Against that, the drug carries a boxed warning about rodent carcinogenicity, and rare nervous-system harms are documented. It is a treatment for a specific infection, not something to take without a reason - the boxed warning itself says unnecessary use should be avoided. (Source 2)

How do you get more of it?

Metronidazole is a prescription-only medicine, not a nutrient, so there is no food or supplement that supplies it and no reason to seek more of it. The amount a person receives is set by a prescriber: the label records, as a position, regimens such as 250 mg three times daily for seven days or a single 2 g dose for trichomoniasis in women. (Source 23)

If it is harmful, what reduces it?

Metronidazole is cleared by the body within about a day of the last dose and does not accumulate as a stored substance. Where a harm has appeared, the literature describes stopping the drug as what reverses it: in a systematic review of 136 published cases of metronidazole-induced encephalopathy, most patients improved significantly once the drug was discontinued. Haemodialysis also removes substantial amounts of the drug and its metabolites. (Source 23)

Why might someone be low in it or missing it?

This question does not apply in the usual sense. Metronidazole is a drug, not a body constituent, so nobody is naturally low in it. Blood levels can be lower than expected if the drug is taken with liver enzyme inducers such as phenytoin or phenobarbital, which speed up its elimination, and the label notes plasma levels are generally lower in men than women because of weight differences. (Source 12)

Which whole foods contain it or feed it?

No whole food contains metronidazole. The food question that does matter runs the other way: the label says alcohol and propylene-glycol-containing products should be avoided during treatment and for three days after, although the controlled human studies behind that warning did not reproduce the reaction. (Source 10)

What happens if you do not have it?

Nothing happens from not having metronidazole unless an infection it treats is present and untreated. Where it is indicated and not given, the untreated infection persists: in the 2018 randomised trial, 19% of women given only a single dose were still positive for Trichomonas four weeks later, and an untreated infection would be expected to persist more often still. For amebic liver abscess the label notes that metronidazole does not remove the need for drainage. (Source 4)

How can you test for it?

There is no routine blood test for metronidazole levels in ordinary care. What is tested is the infection: the label requires the trichomonad to be confirmed by wet smear or culture before treating, and for anaerobic infections it directs culture and susceptibility testing. In practice nucleic acid amplification tests have largely replaced wet mounts for Trichomonas; the 2018 trial used nucleic acid amplification test or culture for its test-of-cure, which is the most reliable confirmation available in the literature we read. (Source 3)

References

1. DailyMed (US National Library of Medicine), SPL from Teva Pharmaceuticals USA, Inc.. Metronidazole Tablets, USP - FDA label, CLINICAL PHARMACOLOGY subsection "Microbiology" (SPL effective 2024-07-01). 2024. Read the source
2. DailyMed (US National Library of Medicine), SPL from Alembic Pharmaceuticals Limited. Metronidazole tablet, film coated, extended release 750 mg - FDA label, boxed WARNING section (SPL effective 2023-01-30). 2023. Read the source
3. DailyMed (US National Library of Medicine), SPL from Teva Pharmaceuticals USA, Inc.. Metronidazole Tablets, USP - FDA label, INDICATIONS AND USAGE subsection "Symptomatic Trichomoniasis" (SPL effective 2024-07-01). 2024. Read the source
4. DailyMed (US National Library of Medicine), SPL from Teva Pharmaceuticals USA, Inc.. Metronidazole Tablets, USP - FDA label, INDICATIONS AND USAGE subsection "Amebiasis" (SPL effective 2024-07-01). 2024. Read the source
5. DailyMed (US National Library of Medicine), SPL from Teva Pharmaceuticals USA, Inc.. Metronidazole Tablets, USP - FDA label, INDICATIONS AND USAGE subsection "Anaerobic Bacterial Infections" (SPL effective 2024-07-01). 2024. Read the source
6. DailyMed (US National Library of Medicine), SPL from Alembic Pharmaceuticals Limited. Metronidazole tablet, film coated, extended release 750 mg - FDA label, INDICATIONS AND USAGE section (SPL effective 2023-01-30). 2023. Read the source
7. Paediatric and perinatal epidemiology. Antibiotic treatment of bacterial vaginosis to prevent preterm delivery: Systematic review and individual participant data meta-analysis.. 2023. PMID 36651636, DOI 10.1111/ppe.12947. Read the source
8. The Cochrane database of systematic reviews. Antibiotic treatment for Clostridium difficile-associated diarrhoea in adults.. 2017. PMID 28257555, DOI 10.1002/14651858.CD004610.pub5. Read the source
9. DailyMed (US National Library of Medicine), SPL from Teva Pharmaceuticals USA, Inc.. Metronidazole Tablets, USP - FDA label, ADVERSE REACTIONS section (SPL effective 2024-07-01). 2024. Read the source
10. DailyMed (US National Library of Medicine), SPL from Teva Pharmaceuticals USA, Inc.. Metronidazole Tablets, USP - FDA label, CONTRAINDICATIONS subsection "Interaction with Alcohol" (SPL effective 2024-07-01). 2024. Read the source
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