Metronidazole

Metronidazole is an antibiotic from a group called nitroimidazoles, and it has a particular talent for killing anaerobic bacteria — the kind that thrive in low-oxygen environments such as the inflamed vaginal lining of BV. Once metronidazole gets inside an anaerobic bacterial cell, the cell's own machinery activates the drug, producing reactive fragments that damage the bacterial DNA. Aerobic cells (including human cells) lack the chemistry to do this, which is why metronidazole leaves the body's normal cells largely alone and why side effects are usually limited. By selectively reducing the anaerobic overgrowth, metronidazole gives the protective lactobacilli room to recover and restore the normal vaginal balance.

This is one of the most useful distinctions to understand, because the two conditions can be confused. BV produces a thin, greyish-white discharge with a characteristic fishy odour, often more noticeable after sex or around the time of a period. Itching and soreness are usually absent or mild. Thrush, on the other hand, is a yeast infection (most often Candida albicans) and tends to produce a thicker, white, "cottage cheese" discharge along with significant itching, burning, and redness. They have different causes and different treatments — antibiotics like metronidazole treat BV but won't help thrush, and antifungals treat thrush but won't help BV. If you're not sure which you have, a brief examination or self-test from a clinician helps make sure the right treatment is chosen.

The standard UK regimen is 400 mg twice a day for five to seven days, taken with food to reduce stomach upset. There's also a single-dose option of 2 g taken once, which is more convenient but has higher rates of recurrence and is generally reserved for situations where compliance is a concern. Tablets should be swallowed whole with a glass of water rather than chewed, because metronidazole is genuinely unpleasant on the tongue and chewing brings out a strong metallic taste.

This is one of the things most patients want to know about, and it's worth understanding rather than just memorising. Metronidazole interferes with one of the enzymes the liver uses to break down alcohol — specifically, an enzyme called acetaldehyde dehydrogenase, which removes a toxic intermediate called acetaldehyde. When that enzyme is blocked and alcohol is consumed, acetaldehyde builds up and produces a very unpleasant set of symptoms: facial flushing, severe nausea and vomiting, headache, a pounding heartbeat, sweating, and a feeling of being acutely unwell. The reaction is sometimes called a "disulfiram-like reaction" because it mirrors what happens when someone on the alcohol-deterrent drug disulfiram drinks. While it's rarely dangerous, it's genuinely awful, and most people who experience it once never want to repeat the experiment.

The standard advice in the UK is to avoid alcohol throughout the course and for at least 48 hours after the last dose, to allow the medicine to clear from your system. Some sources extend this to 72 hours to be cautious. This applies to all forms of alcohol, including beer, wine, spirits, and hidden sources such as cough syrups, mouthwashes containing alcohol, and even some over-the-counter cold remedies. Reading labels matters here — the reaction can be triggered by quite small amounts.

For most people, a small unintentional exposure (a sip of wine, an alcohol-containing mouthwash) doesn't trigger a full reaction, although you may get a mild flush or queasiness. A larger exposure produces the full reaction described above. The treatment is supportive — fluids, rest, and waiting it out, which usually takes a few hours. If symptoms are severe, persistent, or include chest pain or significant breathlessness, seek medical advice promptly, as the reaction can sometimes destabilise other underlying conditions.

The most frequently reported are a metallic or unpleasant taste in the mouth, mild nausea, loss of appetite, abdominal discomfort, and occasional headaches or dizziness. The urine sometimes turns a darker colour during treatment, which is harmless and is just the medicine being processed by the body. Rarer effects include peripheral neuropathy (a tingling or numbness in the hands or feet), which is associated with longer or higher-dose courses rather than the standard five- to seven-day regimen for BV. Persistent or unusual symptoms — particularly any new tingling, severe headache, or rash — should be reviewed by a clinician.

Most women notice an improvement in discharge and odour within two to three days of starting metronidazole, with symptoms typically resolving fully by the end of the course. If symptoms haven't started to improve by day three or four, or if they're getting worse, it's worth checking in with a clinician — sometimes the diagnosis needs revisiting (for example, a co-existing thrush infection or a different cause of discharge).

Yes, in most cases. Despite some old labelling concerns, metronidazole has a substantial safety record in pregnancy, and current UK guidance from the British Association for Sexual Health and HIV (BASHH) supports its use in BV during pregnancy. This matters because untreated BV is associated with a higher risk of preterm birth and late miscarriage, so treatment is genuinely worthwhile. The single 2 g dose is generally avoided in pregnancy and breastfeeding in favour of the longer, lower-dose regimen, because high single doses produce higher peak levels in breast milk. If you're breastfeeding, your prescriber may suggest expressing and discarding milk for 12 to 24 hours after a 2 g dose, but for the standard 400 mg twice-daily regimen, breastfeeding is usually able to continue normally.

A few are worth flagging. Metronidazole increases the effect of warfarin, raising the INR and the risk of bleeding — anyone on warfarin will usually need extra INR checks during and after the course. It can also increase blood levels of lithium, phenytoin, and 5-fluorouracil. Combining metronidazole with disulfiram itself is not recommended because of the risk of psychiatric reactions. As always, tell prescribers about everything you're taking, including herbal remedies and over-the-counter products.

This is one of the most frustrating parts of BV — even after a successful course of treatment, around half of women have a recurrence within twelve months. The reasons are complex, but they include the difficulty of fully restoring the protective lactobacilli, biofilms formed by the offending bacteria, and ongoing exposure to the things that triggered the imbalance in the first place. Practical measures that help include avoiding douching (which strips out the protective flora), avoiding scented soaps, bubble baths, and intimate wipes, washing the external area with plain water or a simple unfragranced wash, and considering condom use, particularly with new partners, since semen can transiently raise vaginal pH. For women who experience frequent recurrences, prescribers sometimes recommend longer or repeat courses of metronidazole, or topical maintenance regimens, depending on the pattern.

For male partners, generally no — BV isn't classified as a sexually transmitted infection in the conventional sense, and treating male partners hasn't been shown to reduce recurrence rates in their female partners. For female partners (in same-sex relationships), the situation is different: simultaneous treatment of both women may be useful, because vaginal flora can be shared. This is worth discussing with a sexual health clinician rather than assuming.

The active ingredient is identical, but the route is different. Tablets distribute the medicine throughout the body, which is effective but produces systemic side effects (taste, nausea, the alcohol interaction, and so on). The vaginal gel — sold under the brand name Zidoval in the UK — places the medicine directly where it's needed, with very little entering the bloodstream. We'll come to Zidoval in detail below, but in short, the gel is often chosen for women who don't tolerate the tablets, who can't avoid alcohol during the course, or who prefer a more localised treatment.