Flagyl: 7 things you should know

Medically reviewed by Carmen Pope, BPharm. Last updated on Aug 31, 2023.

1. How it works

• Flagyl is a brand (trade) name for metronidazole. Metronidazole is an antibiotic that may be used in the treatment of anaerobic infections caused by susceptible bacteria or parasites. Anaerobic means "living without air".
• Once absorbed, Flagyl (metronidazole) diffuses into susceptible organisms, where it becomes active and detrimentally affects DNA synthesis and degradation in the organism, causing cell death. However, the exact way Flagyl works is unknown.
• Flagyl belongs to the class of antibiotics known as nitroimidazoles.

2. Upsides

• Has good penetration into tissue and may be used in the treatment of infections caused by susceptible anaerobic bacteria that occur in the abdomen, gastrointestinal tract, genitourinary area, in the deeper layers of the skin, in bones or joints, throughout the blood, in the central nervous system, heart and lower respiratory tract.
• Active against the following types of bacteria: gram-positive anaerobes (Clostridium species, Eubacterium species, Peptococcus species, Peptostreptococcus species); gram-negative anaerobes (some Bacteroides fragilis group organisms, Fusobacterium species, Protozoal parasites, Entamoeba histolytica, Trichomonas vaginalis).
• May be used in the treatment of symptomatic and asymptomatic T. vaginalis infections. Male sexual partners of people with Trichomoniasis infections should be treated simultaneously; isolating the organism from asymptomatic males can be difficult and negative smears or cultures cannot be relied upon. Treating asymptomatic male partners also reduces the reinfection rate in women.
• May be used in the treatment of acute intestinal amebiasis (amebic dysentery) and amebic liver abscess (in conjunction with pus drainage).
• No dosage adjustment is required in mild-to-moderate kidney or mild-to-moderate liver disease.
• Available as tablets, capsules, extended-release tablets, and in an injectable form.
• Flagyl is available as a generic under the name metronidazole.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

• A headache, nausea, diarrhea, flushing, an unpleasant metallic taste, peripheral neuropathy (nerve damage to fingers and toes), or a rash are common side effects. Rarely, seizures may occur. Elderly people may be more sensitive to the side effects of Flagyl.
• Cases of encephalopathy and peripheral neuropathy have been reported with metronidazole.
• Do not crush, break, or chew extended-release Flagyl tablets. Swallow them whole.
• Flagyl interacts with alcohol and propylene glycol (a food additive). Symptoms include abdominal cramps, nausea, vomiting, headaches, and flushing. Avoid alcohol and products containing propylene glycol while taking Flagyl.
• The dosage of Flagyl may need to be reduced for severe hepatic impairment (Child-Pugh C) and end-stage kidney disease. No dosage adjustment is needed for mild-to-moderate kidney or liver disease.
• May not be suitable for some people including those with a history of certain blood disorders, Cockayne syndrome, Crohn's disease, or with seizure or nerve disorders. Metronidazole will not treat a vaginal yeast infection.
• May interact with several other medicines including disulfiram, warfarin and other anticoagulants, lithium, busulfan, and other drugs metabolized by hepatic enzymes such as CYP2C9. May also cause erroneous results on some laboratory tests, such as aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, and triglycerides.
• Flagyl should not be given to women during the first trimester of pregnancy. Weigh up the risks versus the benefits of giving Flagyl during other trimesters. Flagyl passes into breast milk; however, most studies have reported no adverse effects in infants who were breastfeeding.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

• Flagyl is an antibiotic that is particularly effective at treating infections caused by susceptible anaerobic bacteria and parasites. Avoid alcohol while taking Flagyl and for three days after stopping it.

5. Tips

• May be taken with or without food.
• Take exactly as directed and for the duration intended. For some infections, Flagyl may be given as a single dose. For other types of infections, it may need to be taken for seven to ten days, or even longer. Do not use Flagyl to treat infections other than the one you have been prescribed for. Flagyl has no activity against viruses (such as those that cause a cold or flu).
• Avoid alcohol or products containing propylene glycol (found in some flavored iced teas, frostings, and salad dressings) while taking Flagyl, and for three days after the course of Flagyl has finished.
• Seek medical advice if numbness, pain, or sensitivity occurs in your fingers or toes, or if you notice any skin color changes in these areas. Also, talk to your doctor if you develop a furry-feeling tongue or an inflamed or sore mouth as this may be a sign of Candida overgrowth (a yeast infection).
• Seek urgent medical advice if you develop a rash, diarrhea that is watery or bloody, vision problems, pain when you urinate, or seizures.
• Talk to your doctor or pharmacist before taking any other medication, including that bought over-the-counter, because it may not be compatible with Flagyl. Do not take Flagyl if you have taken disulfiram (Antabuse) within the past two weeks.
• Tell your doctor if you are pregnant or intending to become pregnant because Flagyl tablets may not be suitable for you, and they should not be used during the first trimester of pregnancy. Flagyl is excreted into breast milk so talk to your doctor about breastfeeding while taking Flagyl.

6. Response and effectiveness

• Peak concentrations are reached within one to two hours after dosing; however, it may take up to 48 hours before infection-related symptoms start to abate.

7. Interactions

Medicines that interact with Flagyl may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with Flagyl. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with Flagyl include:

• alcohol
• anticoagulants, such as warfarin
• astemizole
• busulfan
• cimetidine
• disulfiram
• lithium
• medications that inhibit cytochrome p450 enzymes (CYP2C9 and possibly others), such as phenytoin or phenobarbital
• medications that prolong the QT interval (such as amiodarone, chlorpromazine, clarithromycin, haloperidol)
• propylene glycol (a food additive).

Avoid consuming alcohol during treatment with metronidazole and for three days afterward because abdominal cramps, nausea, vomiting, headaches, and flushing may occur.

Note that this list is not all-inclusive and includes only common medications that may interact with Flagyl. You should refer to the prescribing information for Flagyl for a complete list of interactions.

References

• Flagyl [Package Insert]. Revised 08/2023. Pfizer Laboratories Div Pfizer Inc. https://www.drugs.com/pro/flagyl.html

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use flagyl only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

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