Metronidazole

Sounds like 'MET-ro-NYE-da-zole'

Easy-to-read medicine information about metronidazole – what it is, how to take it safely and possible side effects.

Type of medicine Also called
  • Antibiotic
  • Trichozole®
  • Flagyl-S

What is metronidazole?

Metronidazole is an antibiotic that is used to treat infections caused by bugs that do not require oxygen to grow and multiply (called anaerobic infections). Examples of such infections are tooth abscesses, abdominal (stomach) infections, leg ulcers and bacterial vaginal infections. Metronidazole works by killing or stopping the growth of the bugs. Like all antibiotics, metronidazole is not effective against infections caused by viruses. In New Zealand, metronidazole is available as tablets, liquid and suppositories and can be given as an injection in the hospital. 

Dose

  • The dose of metronidazole will be different for different people, depending on the type of infection and your age.
  • The usual dose in adults is 400 milligrams 2 or 3 times a day, but your dose may be more or less than this.
  • Your doctor will advise you on how long to take metronidazole for (usually 3–14 days).
  • For some infections metronidazole can be given a single, larger dose of five 400 mg tablets (2g) to take at once.
  • The dose for children will depend on their body weight. It is usually given 2 or 3 times a day.
  • Always take your metronidazole exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much metronidazole to take, how often to take it, and any special instructions.

How to take metronidazole

If you are unsure about how to take your metronidazole, ask your pharmacist. The following is a guide:

Formulation How to take it
Tablets
  • Take the tablets with food.
  • Swallow the tablets with a glass of water.
Liquid
  • It is best to take the liquid on an empty stomach, if possible, 1 hour before or 2 hours after food.
  • Shake the medicine well.
  • Measure the right amount using an oral syringe or medicine spoon. You can get these from your pharmacy. Do not use a kitchen spoon as it will not give you the right amount. Read more: Tips on how to give medicines to babies and children.
  • Take metronidazole at the same times each day. Space your metronidazole doses evenly through out the day. For example, if you are taking it 3 times a day, take your doses first thing in the morning, early afternoon and at bedtime.

Drinking alcohol

Do not drink alcohol while you are taking metronidazole and for at least one day (24 hours) after finishing treatment. Drinking alcohol with metronidazole may make you feel nauseous (feel sick), vomit (be sick), have stomach cramps, headaches and flushing.

Other tips

  • If you are sick (vomit) less than 30 minutes after having a dose of metronidazole, take the same dose again. But, if you are sick (vomit) and it is more than 30 minutes after having a dose of metronidazole, you do not need to take another dose. Wait until the next normal dose.
  • If you are vomiting and are worried, contact your doctor or HealthLine (0800 611 116) for advice.
  • If you forget to take your dose, take it as soon as you remember. But, if it is nearly time for your next dose, just take the next dose at the right time. Do not take double the dose.
  • It is best to take the whole course of antibiotics for the number of days your doctor has told you to. Do not stop taking it, even if you feel your infection has cleared up. Talk to your doctor first.
  • Store your metronidazole tablets and liquid in a cool, dry place, away from direct heat and light. It does not need to be kept in the fridge.

Precautions – before starting metronidazole

  • Do you have problems with your kidneys or liver?
  • Do you have problems with your brain, spinal cord or nerves? 
  • Do you have a blood disorder?
  • Are you pregnant or breastfeeding?
  • Are taking any other medicines? This includes any medicines you are taking that you can buy without a prescription, as well as herbal and complementary medicines.

If so, it’s important that you tell your doctor or pharmacist before you start metronidazole. Sometimes a medicine isn’t suitable for a person with certain conditions, or it can only be used with extra care.

Possible side effects

Like all medicines metronidazole can cause side effects, although not everyone gets them. Often side effects improve as your body gets used to the new medicine.

Your urine (pee) might change colour
While you are taking metronidazole your urine may look a darker colour than normal. On its own this is nothing to worry about. However, if you also get stomach (tummy) pain, or if you have nausea (feel sick) or feel generally unwell, you should let your doctor know.

Side effects What should I do?
  • Feeling sick (nausea) or vomiting
  • Take metronidazole with food.
  • Do not drink alcohol while you are taking metronidazole.
  • Tell your doctor if troublesome.
  • Headache
  • Dry mouth
  • Food tastes different 
  • These are quite common when you first start taking metronidazole.
  • Tell your doctor if troublesome.
  • Confusion or dizziness
  • Do not drive until it feels safe to do so.
  • Tell your doctor.
  • Vaginal itching, soreness or discharge (thrush)
  • Tell your doctor or pharmacist.
  • Signs of problems with your liver such as yellowing of the skin or eyes, dark urine, pain in the abdomen
  • Tell your doctor immediately or phone HealthLine 0800 611 116. 
  • Signs of an allergic reaction such as skin rash, itching, swelling of your lips, face, and mouth or difficulty breathing
  • Tell your doctor immediately or phone HealthLine 0800 611 116. 

Interactions

Metronidazole may interact with a few medications and herbal supplements, so check with your doctor or pharmacist before starting metronidazole or before starting any new medicines. If you are taking the contraceptive 'pill', the effectiveness of the 'pill' can be reduced if you have a bout of being sick (vomiting) or diarrhoea which lasts for more than 24 hours. If this happens, ask your doctor or pharmacist for advice about contraception over the following few days.

Learn more

Metronidazole New Zealand Formulary Patient Information
Trichazole Medsafe Consumer Information Sheet

References

  1. Metronidazole New Zealand Formulary
  2. Metronidazole New Zealand Formulary for Children
Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 05 Apr 2019