Flagyl antibiotic: uses, dosage and side effects

Here’s everything you need to know about the antibiotic Metronidazole.

Flagyl antibiotic: uses, dosage and side effects
Flagyl – Metronidazole

Medically reviewed by Dr Roger Henderson and words by Helen Marshall, BPharm, MRPharmS

Flagyl is an antibiotic that’s prescribed to treat bacterial and protozoal infections, in particular skin infections, dental infections, pelvic inflammatory disease and bacterial vaginosis.

Here’s everything you need to know about Flagyl, including how it works, who shouldn’t take it and the potential side effects:

What is Flagyl?

Flagyl is an antibiotic that’s prescribed to treat bacterial and protozoal infections, in particular skin infections, dental infections, pelvic inflammatory disease and bacterial vaginosis.

Flagyl comes as tablets (200mg, 400mg and 500mg strengths), liquid, suppositories, vaginal gel, skin creams and skin gels. In hospital it may also be given by drip into a vein (intravenous infusion) for more serious infections or when administration by mouth is not possible.

Flagyl is a brand name for Flagyl tablets and suppositories. Zidoval is a Flagyl vaginal gel. Acea, Anabact, Metrogel, Metrosa, Rosiced, Rozex and Zyomet are all brands of Flagyl skin creams and gels.

What is Flagyl used for?

Flagyl is used to treat infections caused by a wide variety of bacteria known collectively as anaerobic bacteria. These bacteria don’t need oxygen to grow and multiply and can cause infections in areas of the body such as the gut, pelvic cavity, gums and bones. Examples include:

  • Dental infections, eg tooth abscesses and gum infections (ulcerative gingivitis).
  • Skin infections such as infected pressure sores and leg ulcers. Flagyl creams and gels are also used for acne rosacea.
  • A vaginal infection called bacterial vaginosis.
  • Pelvic inflammatory disease.
  • Infections in the abdomen, such as peritonitis, pelvic cellulitis or pelvic abscess.
  • Bowel infections caused by Clostridium difficile bacteria (C.diff).
  • Bone infections (osteomyelitis).
  • Blood infections (septicaemia or blood poisoning).
  • A serious form of pneumonia (necrotising pneumonia).

Flagyl is also used to prevent infection following surgery, particularly gynaecological surgery and surgery on the gut.

Flagyl is used as part of triple therapy to eradicate a type of bacteria called Helicobacter pylori bacteria from the gut in people with peptic ulcers.

Flagyl is also used to treat infections with protozoa. These include Trichomonas vaginalis (which causes trichomonas infection of the vagina) and other protozoa, such as Entamoeba histolytica (which causes amoebic dysentry) and Gardia lambila (which causes giardiasis).

💡 Key facts about Flagyl

Here is some important information about this antibiotic:

● Flagyl is only available on prescription.

● It is suitable for adults and children.

● The dose prescribed, how often to take it and how long for depends on the infection being treated. Always follow your doctor’s instructions.

● Try to space your doses at regular intervals over the day. Always complete the prescribed course.

● The most common side effects are a bitter or metallic taste in your mouth, diarrhoea and feeling sick or vomiting.

● Do not drink alcohol while taking Flagyl, or for at least 48 hours after finishing the course.

● Some liquid medicines such as cough and cold remedies contain small amounts of alcohol and these should also be avoided.

How does Flagyl work?

Flagyl kills bacteria and protozoa by interfering with their genetic material (DNA). It damages their DNA and stops them from forming new DNA. This clears up the infection.

To make sure the micro-organisms causing an infection are susceptible to Flagyl your doctor may take a tissue sample, for example a blood sample or a swab from the infected area.

Flagyl kills bacteria and protozoa by interfering with their genetic material.

Peptic ulcers, which are ulcers in the stomach and upper intestine, can be caused by a type of bacteria called Helicobacter pylori (H pylori). Flagyl is used as part of triple therapy to eradicate these bacteria from the gut. This allows the ulcers to heal and also helps to stop them coming back.

Flagyl is used with another antibiotic (either clarithromycin or amoxicillin) and a proton pump inhibitor such as lansoprazole or omeprazole for triple therapy. The proton pump inhibitors reduce the production of acid in the stomach and help create an environment in which the antibiotics can work more effectively against the bacteria. They also help the ulcer to heal.

Who can’t take Flagyl?

Flagyl is suitable for treating infections in most adults and children. People with liver problems may need a lower dose or extra monitoring.

Don’t take Flagyl if you’re allergic to any ingredients of the medicine. If you feel you have experienced an allergic reaction, stop using Flagyl and inform your doctor or pharmacist immediately.

Flagyl while pregnant

Make sure your doctor knows if you are or think you could be pregnant before you take Flagyl. Flagyl should only be used during pregnancy if considered essential by your doctor. Some infections can be more dangerous during pregnancy than the medicines used to treat them, so your doctor will need to weigh up the risks and benefits of treatment.

Also make sure your doctor knows if you are breastfeeding before you take Flagyl. Flagyl passes into breast milk and could give the milk a bitter taste. Although it’s not generally expected to be harmful, it should only be used if the expected benefit to the mother is greater than any possible risk to the nursing infant. If you do breastfeed while taking a course of Flagyl you should let your doctor know if your baby develops diarrhoea, thrush or nappy rash. Ask your doctor for further advice.

Flagyl dosage

The dose of Flagyl that’s prescribed, how often to take it and how long for, all depend on the type and severity of infection being treated (or prevented), as well as your age and liver function. It is important to follow the instructions given by your doctor. These will be printed on the label that your pharmacist has put on the packet of medicine.

Some infections can be treated with a single dose. For others you might be asked to take a dose two or three times a day, for a course of treatment lasting 3, 5, 7, 10 or 14 days.

Try to space your doses evenly throughout the day. If you forget to take a dose just leave out that dose and take your next dose as usual when it is due. Do not take a double dose to make up for a missed dose.

✔️ Flagyl 200mg, 400mg and 500mg tablets should be taken with or after food or a drink of milk. Swallow the tablets whole with a full glass of liquid; do not crush or chew them.

✔️ If you’ve been prescribed Flagyl liquid, shake the bottle before measuring out the required dose. Use the measuring spoon or oral syringe provided. Don’t use a regular teaspoon or tablespoon, as this will not give an accurate dose. The liquid can be taken either with or without food.

✔️ Flagyl suppositories should be unwrapped before inserting them into the rectum (back passage).

Unless your doctor tells you otherwise, it is important that you finish the prescribed course of Flagyl, even if you feel better or it seems the infection has cleared up. Stopping the course early increases the chance that the infection will come back and that the bacteria will grow resistant to the antibiotic.

You can read more about how to use Flagyl cream, gel or vaginal gel in our pages about the branded products linked at the top of this page.

Can I drink alcohol with Flagyl?

No. It’s important that you don’t drink alcohol while taking Flagyl tablets, liquid, suppositories or vaginal gel, or for at least 48 hours after finishing the prescribed course. Drinking alcohol with this antibiotic can cause unpleasant symptoms such as hot flushes, abdominal cramps, feeling sick, vomiting, headache and a pounding heartbeat (palpitations).

Flagyl side effects

The following are some of the side effects that may be associated with Flagyl. Just because a side effect is listed here doesn’t mean that all people taking Flagyl will experience that or any side effect. Medicines affect people in different ways.

Common Flagyl side effects

  • Feeling sick or vomiting.
  • Diarrhoea.
  • Unpleasant metallic or bitter taste.
  • Sensation of a furry tongue.
  • Sore mouth.
  • Loss of appetite.

Very rare Flagyl side effects

You should contact your doctor straight away if you experience any of these:

  • Allergic reactions such as swelling of lips, face or tongue, skin rashes or itching, wheezing or difficulty breathing.
  • Feeling shaky and unsteady or having problems with speech.
  • Disturbances in your vision.
  • Hallucinations, confusion, high temperature, stiff neck and sensitivity to light.
  • Yellowing of your skin or the whites of your eyes.
  • Unusual bruising, bleeding, high temperature, mouth ulcers or feeling very tired.
  • Read the leaflet that comes with your medicine or talk to your doctor or pharmacist if you want any more information about the possible side effects of Flagyl.
  • If you think you have experienced a side effect, did you know you can report this using the yellow card website?
  • Flagyl and other medicines

Before you take Flagyl, tell your doctor or pharmacist if you’re already taking any medicines, including those bought without a prescription and herbal medicines. Similarly, always check with your doctor or pharmacist before taking any new medicines while you’re taking Flagyl. Some key points are:


If you need to take a painkiller while you’re taking Flagyl it’s fine to take paracetamol or ibuprofen.


Flagyl doesn’t affect hormonal contraceptives such as the pill. However, if you experience vomiting or diarrhoea while taking this antibiotic, this can potentially make your pill less effective at preventing pregnancy. If this happens to you, follow the instructions for vomiting and diarrhoea described in the leaflet provided with your pills.

Medicines that contain alcohol

Some liquid medicines contain small amounts of alcohol and this can be enough to give you a bad reaction while taking Flagyl. It’s important to check the ingredients of liquid medicines before you take them with Flagyl.

Other prescribed medicines

Make sure your doctor or pharmacist know if you’re taking any of the following medicines because you may need some extra monitoring or dose adjustments while you take a course of Flagyl:

  • busulfan
  • capecitabine
  • ciclosporin
  • disulfiram
  • fluorouracil
  • lithium
  • phenobarbital
  • primidone
  • phenytoin
  • warfarin.
  • Vivotif, the oral typhoid vaccine, should not be taken until at least three days after you have finished a course of Flagyl, because the antibiotic could make this vaccine less effective.

Dr Roger Henderson is a Senior GP, national medical columnist and UK medical director for LIVA Healthcare He appears regularly on television and radio and has written multiple books. This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at google.com

By Aquilino Managbanag

Aquilino Managbanag EXPERIENCE: Four years at The Asian Pacific Post, a weekly Canadian newspaper founded in 1993 in Vancouver, British Columbia. The newspaper specialized in reporting Asian issues, and has a readership of 160,000. It has a sister publication in The South Asian Post. EDUCATION: University of Santo Tomas – Manila -- The private Roman Catholic research university is Asia’s oldest existing university. The university takes pride in keeping the Catholic faith and beliefs prominent while holding true to its centuries-old tradition of academic excellence.