Drug & supplement interaction

Fasting + Metronidazole

Metronidazole brand names: Flagyl, Protostat, Metronidazole, Nidazol, Trikacide, and others.

No Known Interaction
Medically reviewed by · Updated July 2026 · Pair last verified Sep 20, 2026
The bottom line No known interaction

We found no known interaction between Fasting and Metronidazole in our sources. Absence of a documented interaction is not the same as proven safety, and does not guarantee one does not exist. Do not start, stop, or change a medication or supplement based on this page; confirm with your pharmacist or doctor first. Additionally, although there is no interaction, the two are separately associated with some of the same side effects, which would make a new symptom harder to attribute to one or the other.

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Interaction report

Fasting and Metronidazole

We checked these two against our interaction database. Here is what we found.

No Known Interaction Found

We did not find a documented interaction between Metronidazole and Fasting in our sources. This does not guarantee that the combination is completely free of risk — research on Fasting alongside specific medications may be limited.

Fasting is an eating pattern rather than something you take. Ask your pharmacist or doctor before changing how you eat while on a prescribed medicine: some medicines are dosed around meals, and a change in diet can change how they work. Do not stop a prescribed medicine on your own.

Always talk with your pharmacist or doctor before making changes to your medications or supplements, particularly if you take several medications or have an ongoing health condition.

Fasting safety at a glance

Fasting overall Use caution Often tolerated by healthy adults but can be risky for some people and medications.
Pregnancy Avoid Prolonged or strict fasting is generally discouraged during pregnancy without medical supervision.
Breastfeeding Use caution May affect hydration, energy, and possibly milk supply, so check with your provider first.

Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

Still wondering about combining Metronidazole and Fasting? Ask Dr. Brian Staiger, PharmD, BCPS directly ›
How we checked

Why No Interaction Is Listed for Metronidazole

Interactions in our licensed database are recorded against drug classes, not just single drug names. Metronidazole is filed under 8 classifications — including cytochrome p450 2c9 (cyp2c9) inhibitors, qt interval-prolonging drugs, cytochrome p450 3a4 (cyp3a4) inhibitors. Fasting has documented interactions with 4 drug classes — the most serious being warfarin (coumadin) (major), anticoagulant/antiplatelet drugs (moderate) and antidiabetes drugs (moderate) — and none of Metronidazole’s classifications are among them.

Classification data comes from our licensed Natural Medicines dataset. A medication can belong to classes beyond those recorded there, so this describes how the check was run — it is not a safety guarantee.

Keep it in perspective

Fasting Does Interact With Other Medications

Fasting has 2 major, 206 moderate known drug interactions in our database — Metronidazole isn’t one of them. If you take anything else, check it against the full list below.

Major 2 Moderate 206 No known interaction 2,622 (incl. Metronidazole)
Taking anything else?

Check Another of Your Medications Against Fasting

Covers the 2,830 medications we’ve checked against Fasting. Pick one to open its full report.

What to actually watch for

No Interaction Found — But Data Suggests Fasting and Metronidazole May Share Several of the Same Side Effects

This is not an interaction. Neither one changes how your body handles the other, and nothing here says they react together. What our sources do show is that each has separately been linked to the effects below — so following Fasting while taking the medicine can make those effects more likely to turn up, and harder to pin on one or the other if they do. One of them is worth acting on rather than waiting out — marked below.

  • An irregular heartbeat Worth acting on Additional detail

    Fasting

    “Dehydration resulting in headache, increased blood pressure, and fever has occurred rarely with medically supervised water-only fasting. Palpitations have occurred rarely. Medically supervised very low-calorie fasting has resulted rarely in mild transitory arrhythmia.”

    Fasting · Natural Medicines adverse-effect record, reported in arrhythmia.

    2 sources cited
    1. 102755. Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complement Altern Med. 2018;18(1):67. PubMed
    2. 102753. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. PubMed

    Metronidazole

    Listed in the ADVERSE REACTIONS section of the FDA label for Metronidazole. Read the label on DailyMed ›

  • Mood changes or irritability Additional detail

    Fasting

    “Medically supervised water-only fasting has resulted in fatigue in about 50% of patients. Insomnia, lightheadedness, and headache were also fairly common. Medically supervised very low-calorie fasting has also resulted rarely in cold sensitivity, headache, dizziness, blurred vision, fatigue, and sleep disturbance.

    Headache, reduced energy, sleep disturbance, cold sensitivity, dizziness, lightheadedness, irritability,…”

    Fasting · Natural Medicines adverse-effect record, reported in cluster headache.

    6 sources cited
    1. 102755. Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complement Altern Med. 2018;18(1):67. PubMed
    2. 102753. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. PubMed
    3. 102737. Tinsley GM, Moore ML, Graybeal AJ, et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr. 2019;110(3):628-640. PubMed
    4. 102741. Cioffi I, Evangelista A, Ponzo V, et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018;16(1):371. PubMed
    5. 102742. Schübel R, Nattenmüller J, Sookthai D, et al. Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial. Am J Clin Nutr. 2018;108(5):933-945. PubMed
    6. 102744. Sundfør TM, Svendsen M, Tonstad S. Effect of intermittent versus continuous energy restriction on weight loss, maintenance and cardiometabolic risk: A randomized 1-year trial. Nutr Metab Cardiovasc Dis. 2018;28(7):698-706. PubMed

    Metronidazole

    Listed in the ADVERSE REACTIONS section of the FDA label for Metronidazole. Read the label on DailyMed ›

  • Fever reported in 5.1% of FDA reports for Metronidazole Additional detail

    Fasting

    Recorded for Fasting in the same Natural Medicines adverse-effect record shown under An irregular heartbeat above.

    Metronidazole

    Listed in the ADVERSE REACTIONS section of the FDA label for Metronidazole, and in voluntary FDA adverse-event reports. Read the label on DailyMed ›

  • Trouble sleeping / Dizziness Additional detail

    Fasting

    Recorded for Fasting in the same Natural Medicines adverse-effect record shown under An irregular heartbeat above.

    Metronidazole

    Listed in the ADVERSE REACTIONS section of the FDA label for Metronidazole. Read the label on DailyMed ›

  • Stomach pain / Diarrhea / Vomiting reported in 6.5% of FDA reports for Metronidazole Additional detail

    Fasting

    “Medically supervised water-only fasting has resulted in nausea in about 32% of patients. Dyspepsia, abdominal pain, diarrhea, and vomiting have occurred less commonly. Vomiting, nausea, diarrhea, dyspepsia, bloating, bad breath, and dry mouth have also occurred rarely with medically supervised very low-calorie fasting.

    The main adverse effect of the intermittent fasting diet is hunger, which generally resolves with t…”

    Fasting · Natural Medicines adverse-effect record, reported in constipation.

    6 sources cited
    1. 102755. Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complement Altern Med. 2018;18(1):67. PubMed
    2. 102753. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. PubMed
    3. 102741. Cioffi I, Evangelista A, Ponzo V, et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018;16(1):371. PubMed
    4. 102748. Conley M, Le Fevre L, Haywood C, Proietto J. Is two days of intermittent energy restriction per week a feasible weight loss approach in obese males? A randomised pilot study. Nutr Diet. 2018;75(1):65-72. PubMed
    5. 108359. Li C, Xing C, Zhang J, Zhao H, Shi W, He B. Eight-hour time-restricted feeding improves endocrine and metabolic profiles in women with anovulatory polycystic ovary syndrome. J Transl Med 2021;19(1):148. PubMed
    6. 108366. He S, Wang J, Zhang J, Xu J. Intermittent versus continuous energy restriction for weight loss and metabolic improvement: A meta-analysis and systematic review. Obesity (Silver Spring) 2021;29(1):108-115. PubMed

    Metronidazole

    Listed in the ADVERSE REACTIONS section of the FDA label for Metronidazole, and in voluntary FDA adverse-event reports. Read the label on DailyMed ›

  • Constipation / Nausea reported in 7.4% of FDA reports for Metronidazole Additional detail

    Fasting

    Recorded for Fasting in the same Natural Medicines adverse-effect record shown under An irregular heartbeat above.

    Metronidazole

    Listed in the ADVERSE REACTIONS section of the FDA label for Metronidazole, and in voluntary FDA adverse-event reports. Read the label on DailyMed ›

Showing the 10 most notable of 12 shared effects — ranked by how unusual each one is, not just whether both lists mention it.

Reported for Fasting but not commonly for Metronidazole: raised blood pressure, blurred vision, high blood sugar and muscle pain.

Effects for Metronidazole come from the ADVERSE REACTIONS section of its FDA label; effects for Fasting come from the Natural Medicines adverse-effect record, quoted above. Where a percentage is shown, it is the share of 64,630 voluntary FDA FAERS reports for Metronidazole that mention that effect — a reporting proportion, not an incidence rate and not evidence the drug caused it. Wording is matched across sources by our own clinical vocabulary, so treat this as a prompt to pay attention, not a diagnosis.

From our Q&A

Questions About Fasting or Metronidazole

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Fasting or Metronidazole, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Metronidazole

This page covers Fasting only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Metronidazole page lists 83 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), American Hellebore (major), Arsenic (major), Cesium (major) and Ephedra (major).

By severity: 7 major · 63 moderate · 13 minor.

From our pharmacist’s overview on the Metronidazole page

“Metronidazole (Flagyl) can stretch the heart's QT interval on its own, and most of the seven major-rated interactions among its 83 flagged supplement ingredients do the same thing, so I read this list with more wariness than most antibiotics earn. Grapefruit and ephedra stand out for adding to that heart-rhythm risk, while activated ch…”

Reviewed by 2 · Jul 18, 2026

Open the Metronidazole interaction page

How much people look this up

Where Fasting and Metronidazole Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Fasting
Top half

#644 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Metronidazole
Top 25%

#565 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Where does our data come from?
  • Source: the Natural Medicines (TRC Healthcare) clinical database — the evidence-graded professional reference for herb & supplement interactions.
  • Citation library: 103,510 references behind our interaction content, 65,311 of them resolved to verified PubMed/DOI sources. See every Fasting reference on this page ↓
  • Freshness: we automatically re-check this combination every week — this pair was last verified September 20, 2026. The Fasting and Metronidazole records this page draws on were last updated July 11, 2026.
  • What “no known interaction” means: no documented interaction in the sources above — not a guarantee of safety. Supplements are studied far less than prescription drugs, so we say exactly what the evidence supports, and nothing more.
Data structure & sourcing designed and verified by .
Eating pattern

About Fasting

What is Fasting?

  • Weight loss
  • Blood sugar control
  • Heart health
  • Religious or spiritual practice
  • General wellness and longevity

Fasting means voluntarily going without food (and sometimes drink) for set periods. Some research suggests it may help with weight loss and blood sugar in certain people, but results vary and quality of evidence is mixed. It is not safe for everyone, so check with your doctor or pharmacist before starting—especially if you take medication or have a health condition.

Not applicable (a dietary practice, not a substance) · Time-restricted eating, alternate-day fasting, periodic multi-day fasts, intermittent fasting (e.g., 16:8 or 5:2), religious fasting

An interaction check asks whether these two react with each other — it does not ask whether Fasting is a good idea for someone in your situation. Here is what our Fasting record says about it on its own.

Fasting: dosing

There is no single "correct" fasting schedule, and no standardized dose. Popular approaches include daily time-restricted eating (such as a 16:8 pattern), the 5:2 plan, and alternate-day fasting. The best choice depends on your health, lifestyle, and goals.If you are new to fasting, it's wise to start gently—such as a slightly shorter eating window—and to drink plenty of water during fasting periods (unless your fast restricts fluids for religious reasons).

From our Fasting monograph — the full record covers uses, side effects and every documented interaction.

Read the full Fasting monograph & all interactions
Medication

About Metronidazole

What Is Metronidazole Used For?

Brand names: Flagyl, Protostat, Metronidazole, Nidazol, Trikacide, MetroGel, Flagyl IV, Metronidazole Redi-Infusion, Flagyl IV RTU, Metro IV, Anabact, Metrosa, Metrotop, Noritate, Rozex Cream, Rozex Gel, Zyomet, Likmez

Metronidazole treats three main types of infections. It's used for trichomoniasis (a sexually transmitted parasitic infection), amebiasis (an intestinal or liver infection caused by a parasite), and serious anaerobic bacterial infections — infections caused by bacteria that thrive without oxygen, which can affect the abdomen, skin, bones, lungs, and other areas.

Because sexual partners can pass trichomoniasis back and forth, both partners are typically treated at the same time, even if one has no symptoms.

Most-Reported Side Effects for Metronidazole

From FAERS, the FDA’s public side-effect reporting system (64,630 reports, all brands). Reports are voluntary and are not proof the drug caused the effect.

Nausea 4,812 reports
Diarrhea 4,197 reports
Vomiting 3,393 reports
Fever 3,294 reports
Drug hypersensitivity 3,160 reports
See full Metronidazole drug information
Keep checking

Other Drugs & Fasting

Browse every other medication we’ve checked against Fasting — both the ones with a known interaction and the ones without. Select any drug for the full detail.

2Major
206Moderate
0Minor
2,621No known interaction
2 Major 206 Moderate 2,621 No interaction
Major

2 interactions

Moderate

206 interactions

Showing 60 of 206 moderate interactions — see all 206 on the Fasting page.

No interaction 2,621 drugs checked — show the list

We also checked Fasting against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,621 medications checked with no known interaction — see the full Fasting list.

See the full Fasting interactions page
See every supplement checked against Metronidazole, ranked by severity
Evidence & sources

Fasting References

The 23 references that drive our Fasting monograph and interaction data, from the evidence-graded Natural Medicines (TRC Healthcare) database. Citations with a link open the study on PubMed or the publisher’s site.

  1. Carter S, Clifton PM, Keogh JB. Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with Type 2 diabetes: A randomized noninferiority trial. JAMA Netw Open. 2018;1(3):e180756. PubMed
  2. Tinsley GM, Moore ML, Graybeal AJ, et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr. 2019;110(3):628-640. PubMed
  3. Cioffi I, Evangelista A, Ponzo V, et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018;16(1):371. PubMed
  4. Schübel R, Nattenmüller J, Sookthai D, et al. Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial. Am J Clin Nutr. 2018;108(5):933-945. PubMed
  5. Sundfør TM, Svendsen M, Tonstad S. Effect of intermittent versus continuous energy restriction on weight loss, maintenance and cardiometabolic risk: A randomized 1-year trial. Nutr Metab Cardiovasc Dis. 2018;28(7):698-706. PubMed
  6. Harris L, Hamilton S, Azevedo LB, et al. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep. 2018;16(2):507-547. PubMed
  7. Harris L, McGarty A, Hutchison L, Ells L, Hankey C. Short-term intermittent energy restriction interventions for weight management: a systematic review and meta-analysis. Obes Rev. 2018;19(1):1-13. PubMed
  8. Conley M, Le Fevre L, Haywood C, Proietto J. Is two days of intermittent energy restriction per week a feasible weight loss approach in obese males? A randomised pilot study. Nutr Diet. 2018;75(1):65-72. PubMed
  9. Chowdhury A, Khan H, Lasker SS, Chowdhury TA. Fasting outcomes in people with diabetes and chronic kidney disease in East London during Ramadan 2018: The East London diabetes in Ramadan survey. Diabetes Res Clin Pract. 2019;152:166-170. PubMed
  10. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. PubMed
  11. Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complement Altern Med. 2018;18(1):67. PubMed
  12. Katada Y, Nakagawa S, Nishimura A, et al. Effects of fasting on warfarin sensitivity index in patients undergoing cardiovascular surgery. Eur J Clin Pharmacol. 2019;75(4):561-568. PubMed
  13. Lai YF, Cheen MHH, Lim SH, et al. The effects of fasting in Muslim patients taking warfarin. J Thromb Haemost. 2014;12(3):349-54. PubMed
  14. Grajower MM, Horne BD. Clinical management of intermittent fasting in patients with diabetes mellitus. Nutrients. 2019;11(4):873. PubMed
  15. Ibrahim M, Davies MJ, Ahmad E, et al. Recommendations for management of diabetes during Ramadan: update 2020, applying the principles of the ADA/EASD consensus. BMJ Open Diabetes Res Care. 2020;8(1):e001248. PubMed

Medication overviews come from MedlinePlus (NIH); supplement label data from the NIH Dietary Supplement Label Database. Content is written and reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

Pharmacist Counseling Corner

Following Fasting While Taking Metronidazole: Common Questions

Is it safe to follow Fasting while taking Metronidazole?
We do not list a known interaction between Fasting and Metronidazole. That does not guarantee the combination is risk-free, so confirm with your pharmacist or doctor — especially if you take other medications or have a health condition.
Do Fasting and Metronidazole have any of the same side effects?
Yes — our sources link each of them separately to 10 of the same effects. That is not an interaction: neither changes how your body handles the other. It does mean that if one of those effects turns up, it can be harder to tell which one is behind it. They are listed on this page with the source behind each.
How was the Fasting and Metronidazole combination checked?
Interactions in our licensed database are recorded against drug classes. Metronidazole is filed under 8 classifications, and Fasting has documented interactions with 4 drug classes — none of Metronidazole's classifications are among them. A medication can belong to classes beyond those recorded in our dataset, so this describes how the check was run rather than guaranteeing safety.
Does Fasting interact with any other medications?
Yes. Fasting has 208 documented drug interactions in our database — Metronidazole is simply not one of them. If you take anything else, it is worth checking each one; they are all listed on this page.
Does Metronidazole interact with any supplements?
Yes. Metronidazole has documented interactions with 83 supplements in our database — Fasting is not one of them. The Metronidazole interaction page linked on this page lists every one, ranked by severity.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Where next

Keep Exploring

Fasting monograph Uses, safety profile, and every drug interaction in one place Metronidazole drug information The full plain-English guide to this medication Metronidazole supplement interactions Every supplement checked against Metronidazole, ranked by severity Supplement interaction checker Check any supplement or product against your medications Browse medications A–Z Every drug we’ve checked against herbs & supplements References The 23 citations behind this page Ask a pharmacist Free, personal answer from our licensed team

Still Have a Question About Metronidazole and Fasting?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.