Bacterial vaginosis is a common vaginal condition caused by an imbalance in the natural bacteria found in the vagina. The vagina normally contains a delicate balance of different bacteria, predominantly 'good' bacteria called lactobacilli, which maintain an acidic pH to protect against infection. When this balance is disrupted and harmful anaerobic bacteria overgrow, BV occurs. It is the most common cause of abnormal vaginal discharge in women of childbearing age, and affects up to 29% of women worldwide. Many women with BV have no symptoms at all and do not require treatment. Those who do experience symptoms may notice a thin, grey or white watery discharge and a characteristic fishy smell, which is often more noticeable after sex. BV does not typically cause soreness, itching, or burning — these symptoms are more likely to indicate a different infection such as thrush or an STI.

BV is not a sexually transmitted infection. Any woman can develop BV, including those in same-sex relationships and women who have never been sexually active. However, sexual activity can be a trigger. Unprotected sex can disrupt the vaginal pH balance, as semen is alkaline and can interfere with the vagina's natural acidity. BV can be passed between female sexual partners. It is not possible, however, for a woman to pass BV to a male partner. Despite not being an STI itself, having BV can increase your risk of acquiring sexually transmitted infections such as chlamydia, HIV, and herpes, as the normal protective acidity of the vagina is reduced.

Metronidazole is the first-line treatment for bacterial vaginosis recommended by UK clinical guidelines. It is highly effective, with studies showing cure rates of between 70% and 95% when a full course is completed correctly. The 7-day twice-daily regimen (400mg twice a day) is considered the most effective oral option. The single large dose of 2g (five 400mg tablets taken at once) has been shown in clinical trials to be slightly less effective at clearing BV than a 5- or 7-day course, and is more likely to cause side effects. Most women begin to feel symptom relief within the first few days of starting treatment.

Most people begin to notice an improvement in their BV symptoms — such as a reduction in discharge and odour — within the first 2 to 3 days of starting metronidazole. A full 7-day course should clear most cases of BV completely. You should continue to take the full course even if your symptoms improve before it is finished, as stopping early risks the infection returning. If your symptoms have not improved within 2 weeks of completing treatment, or if they return, you should speak to your doctor to rule out other possible causes or to discuss a repeat course.

The standard adult dose for treating bacterial vaginosis with metronidazole 400mg tablets is one tablet (400mg) taken twice a day, approximately 12 hours apart, for 7 days. Always swallow the tablets whole with a full glass of water — do not chew them. Take each dose during or just after a meal to help reduce the risk of stomach upset. Try to take your doses at the same times each day to maintain consistent drug levels in your body. An alternative single high-dose regimen of 2g (five 400mg tablets taken all at once) exists, but this is considered slightly less effective and more likely to cause nausea.

Yes — it is essential to complete the full prescribed course of metronidazole, even if your BV symptoms improve or disappear before the course is finished. Stopping treatment early means the infection may not be fully cleared and significantly increases the risk of it returning. Incomplete antibiotic courses can also contribute to antibiotic resistance. Always take all tablets as directed by your prescriber.

No — you must not drink alcohol while taking metronidazole, and for at least 48 hours after finishing your course. Mixing metronidazole with alcohol can cause a severe and very unpleasant reaction sometimes called a 'disulfiram-like reaction'. This can result in nausea, vomiting, flushing of the skin, rapid heartbeat (palpitations), throbbing headache, and breathlessness. In very rare cases, the combination can be serious. Even small amounts of alcohol in products such as mouthwash, cough syrup, or some food sauces can trigger this reaction, so check labels carefully. It is not enough to reduce your alcohol intake — complete abstinence is required for the duration of treatment and for 2 days after your final dose.