Metronidazole (Flagyl suppositories)

How does it work?

Flagyl suppositories contain the active ingredient metronidazole, which is a type of medicine called an antibiotic. (NB. Metronidazole is also available without a brand name, ie as the generic medicine.) Metronidazole is used to treat infections caused by bacteria and other micro-organisms called protozoa.

Metronidazole works by entering bacterial and protozoal cells and interfering with their genetic material (DNA). It damages the DNA and also prevents the bacteria and protozoa from forming new DNA. This ultimately results in metronidazole killing the micro-organisms, which clears up the infection.

Metronidazole kills a wide variety of bacteria that are known collectively as anaerobic bacteria. This means that they do not need oxygen to grow and multiply. Anaerobic bacteria can cause infections in areas of the body such as the bones, gut, pelvic cavity and gums. Metronidazole is used to treat these types of infections.

Metronidazole is also used to prevent infection following surgery, particularly gynaecological surgery and surgery on the gut, where many anaerobic bacteria may be found. In high doses, metronidazole penetrates the brain and can be used to treat abscesses in the brain.

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

Flagyl suppositories are used when administration of metronidazole by mouth is not possible. The metronidazole is absorbed into the bloodstream from the rectum.

What is it used for?

  • Preventing and treating infections with anaerobic bacteria following surgery, particularly surgery on the gut or gynaecological surgery.
  • Bacterial infection of the blood (septicaemia or blood poisoning).
  • Bacterial infection of the blood following childbirth (puerperal fever/sepsis).
  • A serious form of pneumonia (necrotising pneumonia).
  • Bacterial infections of bone (osteomyelitis).
  • Bacterial infections in the abdomen (peritonitis).
  • Abscess in the pelvic cavity, usually a result of an abdominal infection.
  • Inflammation of the connective tissue and muscle around the uterus due to bacterial infection (pelvic cellulitis).
  • Brain abscesses.

How do I use it?

  • The dose of metronidazole that is prescribed, how often to use it and for how long, 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 dispensing label that your pharmacist has put on the packet of medicine.
  • Flagyl suppositories are to be inserted into the rectum only. They should not be taken by mouth.
  • Unless your doctor tells you otherwise, it is important that you finish the prescribed course of this antibiotic medicine, 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.


  • Do not drink alcohol while using this antibiotic and for at least 48 hours after finishing the course, as this can cause unpleasant symptoms such as hot flushes, abdominal cramps, nausea, vomiting, headache and palpitations.
  • This medicine may cause various side effects that could impair your mental or physical ability to safely drive or operate machinary. Altough these are very rare, you should be aware of how you react to this medicine before driving or operating machinary.
  • If your course of treatment is longer than 10 days it is recommended that you are monitored by your doctor, who may carry out tests to check for side effects and that the medicine is working properly. Tell your doctor if you experience any pins and needles type sensations, unsteadiness walking, dizziness, fits, severe abdominal pain, or yellowing of your skin or whites of your eyes (jaundice) while using the medicine.

Use with caution in

  • Decreased liver function.
  • Brain disease that has occured as a result of liver disease (hepatic encephalopathy).
  • People with diseases of the brain or spinal cord.
  • Hereditary blood disorders called porphyrias.

Not to be used in

  • Known sensitivity or allergy to any ingredient.

This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy. If you feel you have experienced an allergic reaction, stop using this medicine and inform your doctor or pharmacist immediately.

Pregnancy and breastfeeding

Certain medicines should not be used during pregnancy or breastfeeding. However, other medicines may be safely used in pregnancy or breastfeeding providing the benefits to the mother outweigh the risks to the unborn baby. Always inform your doctor if you are pregnant or planning a pregnancy, before using any medicine.

  • The safety of this medicine during pregnancy has not been fully established. It should only be used during pregnancy if considered essential by your doctor. The manufacturer recommends that high dosage regimens should be avoided. Seek further medical advice from your doctor.
  • This medicine passes into breast milk. It should be used with caution in breastfeeding mothers and only if the expected benefit to the mother is greater than any possible risk to the nursing infant. High dosage regimens should be avoided unless the mother stops breastfeeding during the course. Seek further medical advice from your doctor.

Side effects

Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects that are known to be associated with this medicine. Just because a side effect is stated here, it does not mean that all people using this medicine will experience that or any side effect. The majority of these side effects are very rare.

  • Disturbances of the gut such as diarrhoea, nausea, vomiting or abdominal pain.
  • Unpleasant taste.
  • Sensation of a furry tongue.
  • Inflamed and sore mouth.
  • Loss of appetite.
  • Severe swelling of lips, face or tongue (angioedema).
  • Drowsiness.
  • Dizziness.
  • Headache.
  • Shaky movements and unsteady walk (ataxia).
  • Skin rashes or itching.
  • Pain in the muscles or joints.
  • Darkening of the urine.
  • Visual disturbances.
  • Confusion.
  • Depressed mood.
  • Hallucinations.
  • Liver disorders.
  • Disorder of the peripheral nerves called peripheral neuropathy, that causes weakness and numbness (on prolonged or intensive therapy).
  • Decrease in the number of white blood cells in the blood (leucopenia).
  • Seizures.

The side effects listed above may not include all of the side effects reported by the medicine's manufacturer. For more information about any other possible risks associated with this medicine, please read the information provided with the medicine or consult your doctor or pharmacist.

How can this medicine affect other medicines?

It is important to tell your doctor or pharmacist what medicines you are already taking, including those bought without a prescription and herbal medicines, before you start treatment with this medicine. Similarly, check with your doctor or pharmacist before taking any new medicines while using this one, to ensure that the combination is safe.

You should avoid alcohol while using this antibiotic, and for at least 48 hours after finishing the course, as this can cause unpleasant symptoms such as hot flushes, abdominal cramps, nausea, vomiting, headache and palpitations.

Metronidazole may enhance the anti-blood-clotting effect of anticoagulant medicines such as warfarin and nicoumalone, and this may increase the risk of bleeding. If you are taking an anticoagulant medicine your blood clotting time (INR) should be monitored during treatment with a course of metronidazole, and your doctor may need to reduce your anticoagulant dose.

Metronidazole may increase the blood levels of the following medicines:

  • busulfan
  • ciclosporin (people taking ciclosporin may need their ciclosporin blood level and kidney function monitored while having a course of metronidazole)
  • fluorouracil (also known as 5-FU, this is an anticancer medicine; its side effects are increased by metronidazole, but not its anti-cancer effects)
  • lithium (people taking lithium should have their lithium blood level and kidney function monitored while having a course of metronidazole)
  • phenytoin.

Barbiturates, such as the antiepileptic medicine phenobarbital, decrease the blood level of metronidazole and may make it less effective at treating infection. Your doctor may prescribe you a larger than normal dose of metronidazole if you are taking a barbiturate.

If metronidazole is used by people taking disulfiram there may be a risk of confusion and psychotic symptoms such as paranoia and hallucinations. This combination should be avoided where possible or closely monitored if essential.

Oral typhoid vaccine (Vivotif) should not be taken until at least three days after you have finished a course of this antibiotic, because the antibiotic could make the vaccine less effective.

In the past, women using hormonal contraception such as the pill or patch would be advised to use an extra method of contraception (eg condoms) while having treatment with an antibiotic like this one and for seven days after finishing the course. However, this advice has now changed. You no longer need to use an extra method of contraception with the pill, patch or vaginal ring while you have a course of antibiotics. This change in advice comes because to date there is no evidence to prove that antibiotics (other than rifampicin or rifabutin) affect these contraceptives. This is the latest guidance from the Faculty of Sexual & Reproductive Healthcare.

However, if you are taking the contraceptive pill and experience vomiting or diarrhoea as a result of treatment with this antibiotic, you should follow the instructions for vomiting and diarrhoea described in the leaflet provided with your pills.

Other medicines containing the same active ingredient

Oral and rectal brands of metronidazole:

Flagyl S Flagyl tablets Metrolyl suppositories

Metronidazole tablets, suspension and injection are also available without a brand name, ie as the generic medicine.

Brands of metronidazole that are applied to the skin:

Acea gelAnabact gelMetrogel gel
Metrosa gel Metrotop gel Rosiced cream
Rozex cream/gel Zyomet gel

Brands of metronidazole for application to the vagina:

Zidoval vaginal gel