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How do metronidazole tablets work?

Metronidazole is an antibiotic which works most effectively against anaerobic bacteria. Anaerobic bacteria do not require oxygen to live. When a woman has bacterial vaginosis (BV), anaerobic bacteria replace the lactobacilli which usually maintain a healthy vaginal environment. Metronidazole kills the anaerobic bacteria allowing the lactobacilli to take over again and restore the natural balance in the vagina.

Dosage regime for metronidazole 400mg tablets

There are many different regimes for taking metronidazole tablets, depending on the infection being treated. For BV, there are also a few possible regimes. Dr Fox supplies 14 x 400mg tablets to be taken twice daily 12 hours apart for 7 days - swallow whole with water, with a meal.

Note: metronidazole 400mg is supplied from the manufacturers in boxes of 21 tablets. You will receive an opened or blank box with 14 tablets only.

Alternative treatments for bacterial vaginosis

There are several alternative prescription treatments for BV. Always use medication according to the regime actually prescribed for you.

Metronidazole tablets

• 400mg tablets twice a day for 5-7 days.
• 500mg tablets twice a day for 5-7 days.
• One dose of 2 grams (5 x 400mg tablets or 4 x 500mg tablets) all at once.

Clindamycin tablets

• 300mg tablets twice a day for 7 days.
• Usually reserved for pregnancy.

Metronidazole 0.75% vaginal gel (Zidoval)

Every night for 5 nights.

Clindamycin 2% vaginal cream (Dalacin)

Every night for 7 nights.

Most treatments have been shown to be equally effective with 70-80% cure rates. However the large one-off dose of 2 grams of oral metronidazole was slightly less effective in trials and is more likely to cause side effects.

Cautions when taking metronidazole 400mg tablets

• Metronidazole should not be used in severe liver failure.
• Metronidazole may aggravate neurological diseases.
• Kidney disease or dialysis may require altered dosing - specialist advice required.
• Manufacturers advise not to drink alcohol whilst taking metronidazole 400mg tablets and for the 48 hours afterwards. Taking alcohol with metronidazole may cause a 'disulfiram reaction'. Disulfiram is given to people with alcoholism, to help them to stop drinking alcohol. It causes a bad reaction when alcohol is taken. The disulfiram reaction includes nausea, vomiting, dizziness, chest, head and abdominal pains. See also FAQs.

Metronidazole may interact with these other medications:

• Busulphan.
• Cancer chemotherapy.
• Ciclosporin.
• Disulfiram.
• Fluorouracil.
• Lithium.
• Phenobarbital.
• Phenytoin.
• Warfarin type anticoagulants (requiring regular blood tests).

Checks for suitability are carried out in the online consultation and full details are in the manufacturer's patient information leaflet supplied with your medication.

Metronidazole tablets and pregnancy

Metronidazole has been used extensively in pregnancy and has not been shown to cause birth defects, stillbirth, low birth weight, or preterm delivery. Some studies have linked metronidazole use with increased miscarriage. However the increase may have been caused by the infection being treated, not the metronidazole.

The single high dose of metronidazole 2 grams is not recommended in pregnancy. Dr Fox does not prescribe for pregnant women. Please consult your GP or midwife.

See also BUMPS - Metronidazole.

Metronidazole tablets and breastfeeding

It is not usually recommended to take metronidazole tablets whilst breastfeeding. Metronidazole passes into breast milk and will also affect its taste. Dr Fox does not prescribe for women who are breastfeeding. Please consult your GP.

Possible side effects

As with all medications, there is a risk of side effects but not everyone gets them and they are less likely when taking a short course of a few days treatment.

Very rarely, drowsiness, dizziness and visual disorders may occur whilst taking metronidazole. If affected do not drive, cycle or operate other machinery.

Mild side effects of metronidazole can include nausea, vomiting, diarrhoea, loss of appetite, numbness or tingling, metallic or unpleasant taste in mouth, skin dryness, discomfort, or itch.

Metronidazole may make urine appear darker.

Rare conditions, when you should stop metronidazole immediately and contact a doctor:

• Acute allergy - signs include: swelling of the hands, feet, ankles, face, lips or throat, which may cause difficulty in swallowing or breathing, with an associated itchy, lumpy, nettle rash (hives/urticaria).
• Other severe skin rashes with a high temperature - possible Stevens-Johnson Syndrome.
• Fever, stiff neck, headache, hallucinations, confusion, difficulty using arms or legs or speaking - possible encephalopathy (brain inflammation).

More information on side effects can be found in the manufacturer's patient information leaflet supplied with your medication.

General advice to relieve symptoms and reduce risk of getting bacterial vaginosis

• Take a shower instead of a bath.
• Use only water and a plain soap or aqueous cream to wash genital area.
• Do not use vaginal douching, vaginal washes, vaginal deodorants.
• Do not use antiseptic, perfumed soap, bubble baths, shampoo, or shower gel in the bath.
• Do not use strong detergent to wash underwear.
• Do not smoke.

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