Flagyl for Bacterial Infections

What to Know About the Broad-Spectrum Antibiotic

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Flagyl, known generically as metronidazole or by the brand name Protostat, is an antibacterial drug that is classified as a broad-spectrum antibiotic. It is used to kill any bacteria in the body that are causing infection. In some cases, Flagyl may be used to treat infections or other complications of IBD. It might be given alternately or at the same time as ciprofloxacin, another type of antibiotic.

Why Flagyl Is Prescribed

Flagyl is used to fight off, or to prevent, a bacterial infection. It is used to treat a wide variety of infections, including those in the abdomen, bones, joints, nervous system, respiratory tract, and skin, as well as vaginal and intestinal infections.

Who Should Not Take Flagyl 

People with an allergy or sensitivity to metronidazole should also not take Flagyl. Flagyl should be used under the close supervision of a physician by anyone who has liver disease. Yeast infections may worsen while taking Flagyl.

How Flagyl Is Taken

When taken at home to treat an infection, Flagyl is taken orally in tablet form. In order for Flagyl to effectively kill harmful bacteria, it is important to maintain a constant level of the drug in the blood.

Therefore, it must be taken at regular intervals without missing any doses. Take Flagyl with 8 ounces of water. Flagyl can be taken by itself, or it can be taken with meals. If dry mouth becomes bothersome, try chewing gum, or sucking on hard candy or ice chips.

If A Dose Is Missed

When a dose is missed, take it as soon as it is remembered. If the next dose should be taken soon, just take that dose. Don't double up or take more than one dose of Flagyl at a time.

Stopping Flagyl

Do not stop Flagyl before taking all the doses. After a few days of treatment, most people start to feel better, but that does not mean the infection is entirely gone. Take all of the medication that was prescribed unless instructed to stop taking it by a healthcare professional.

Stopping the medication before the bacterial infection is completely gone can result in serious consequences. A stronger strain of bacteria may develop, or the infection may come back again and be more difficult to treat.

Side Effects

The most serious side effects from Flagyl are seizures and tingling or numbness in the extremities (arms, legs, hands, and feet). If these symptoms start, stop taking Flagyl and call a doctor immediately. Other side effects include abdominal cramping, diarrhea, headache, nausea, loss of appetite, and vomiting.

The Effect of Flagyl on IBD Diarrhea

Antibiotics kill off bacteria in the body, and many can't distinguish between "good" and "bad" bacteria. Therefore, with some antibiotics, "good" bacteria in the colon may be killed along with the "bad," resulting in diarrhea. However, Flagyl doesn't tend to cause antibiotic-associated diarrhea—in fact, it may be a treatment for it.

Medication Interactions

Flagyl could interact with several drugs. Tell the prescribing physician about all drugs and nutritional supplements, especially those from the following list, which may interact with Flagyl.

  • Alcohol
  • Amprenavir
  • Anticoagulants (such as Warfarin)
  • Barbiturate medicines for inducing sleep or treating seizures (convulsions)
  • Carbamazepine
  • Cimetidine
  • Disulfiram (Antabuse)
  • Dofetilide
  • Fluorouracil
  • Lithium
  • Methadone
  • Phenytoin
  • Sirolimus
  • Tacrolimus

Food Interactions

Alcoholic beverages should not be consumed until 72 hours (3 days) after the last dose of Flagyl is taken. Consuming alcohol while taking Flagyl could result in abdominal cramps, nausea, vomiting, headaches, and flushing. Flagyl can also change the taste of alcohol. Take care to avoid alcohol from unexpected sources, such as over-the-counter cough suppressants or cold products (NyQuil, for example).

Safety During Pregnancy

Pregnant women in their first trimester should avoid Flagyl. The FDA has classified Flagyl as a type B drug. The effect that Flagyl has on an unborn child has not been studied extensively. Flagyl should only be used during pregnancy if it is clearly needed. Notify the prescribing doctor if you become pregnant while you are taking Flagyl. Flagyl does pass into breast milk and could affect a nursing infant. It should be used with care in nursing mothers.

Combining With Milk Thistle

Milk thistle (Silybum marianum) may help to protect the liver from medications that may harm it, such as Flagyl. It has not been studied in direct correlation with Flagyl, but it may be prescribed as a complementary therapy.

A Word From Verywell

Flagyl is a common antibiotic that is used to treat a variety of conditions for several decades. Some people do experience side effects, but they are often manageable. For side effects that become troublesome or can't be managed, a doctor should be contacted right away.

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Article Sources

  1. Howe K, Kissinger PJ. Single-Dose Compared With Multidose Metronidazole for the Treatment of Trichomoniasis in Women: A Meta-AnalysisSex Transm Dis. 2017;44(1):29–34. doi:10.1097/OLQ.0000000000000537

  2. Nitzan O, Elias M, Peretz A, Saliba W. Role of antibiotics for treatment of inflammatory bowel diseaseWorld J Gastroenterol. 2016;22(3):1078–1087. doi:10.3748/wjg.v22.i3.1078

  3. Hernández ceruelos A, Romero-quezada LC, Ruvalcaba ledezma JC, López contreras L. Therapeutic uses of metronidazole and its side effects: an update. Eur Rev Med Pharmacol Sci. 2019;23(1):397-401.

  4. Cudmore SL, Delgaty KL, Hayward-McClelland SF, Petrin DP, Garber GE. Treatment of infections caused by metronidazole-resistant Trichomonas vaginalisClin Microbiol Rev. 2004;17(4):783–793. doi:10.1128/CMR.17.4.783-793.2004

  5. Weir CB, Le JK. Metronidazole. [Updated 2019 May 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539728/

  6. Agarwal A, Kanekar S, Sabat S, Thamburaj K. Metronidazole-Induced Cerebellar ToxicityNeurol Int. 2016;8(1):6365. Published 2016 Apr 1. doi:10.4081/ni.2016.6365

  7. Cina SJ, Russell RA, Conradi SE. Sudden death due to metronidazole/ethanol interaction. Am J Forensic Med Pathol. 1996;17(4):343-6.

  8. Teicher MH, Altesman RI, Cole JO, Schatzberg AF. Possible nephrotoxic interaction of lithium and metronidazole. JAMA. 1987;257(24):3365-6.

  9. Sheehy O, Santos F, Ferreira E, Berard A. The use of metronidazole during pregnancy: a review of evidence. Curr Drug Saf. 2015;10(2):170-9.

  10. Kawaguchi-Suzuki M, Frye RF, Zhu HJ, et al. The effects of milk thistle (Silybum marianum) on human cytochrome P450 activityDrug Metab Dispos. 2014;42(10):1611–1616. doi:10.1124/dmd.114.057232

Additional Reading

  • Pfizer. "FLAGYL U.S. Physician Prescribing Information." Pfizer Inc. Apr 2010. 24 Jan 2012.
  • The American Society of Health-System Pharmacists, Inc. "Metronidazole Oral." MedLine Plus 1 Sept 2008. 24 Jan 2012.