Bacterial vaginosis: symptoms, treatment, recurrence

What is bacterial vaginosis?

The vagina normally contains a flora dominated by lactobacilli, which maintain an acidic environment (pH ≤ 4.5) and protect against infection. In bacterial vaginosis, these lactobacilli decrease and other bacteria, mainly anaerobic ones (such as Gardnerella), multiply. This is called dysbiosis: it is not an infection by a single outside microbe, but an imbalance.

It is the most common cause of abnormal vaginal discharge in women of reproductive age. According to the StepUp trial, it affects around a third of women of reproductive age worldwide; its frequency varies greatly between populations. In France, it is estimated at around 7% in the first trimester of pregnancy, according to the guidelines of the CNGOF (French College of Gynaecologists and Obstetricians).

What are the symptoms?

• vaginal discharge that is thin, uniform, greyish-white;
• fishy odour, more noticeable after intercourse or during periods;
• itching and burning are rare, unlike with thrush.

A significant proportion of women have no symptoms at all. Vaginosis without symptoms, outside particular situations (high-risk pregnancy, gynaecological procedure), does not generally require treatment.

See also "Vaginal discharge (leucorrhoea): causes, symptoms, when to see a doctor" and "Recurrent vaginal thrush: causes and treatments".

How is it diagnosed?

The diagnosis is based on the gynaecological examination and a vaginal swab:

• Amsel criteria (at least three out of four): uniform discharge, vaginal pH above 4.5, fishy odour after adding potassium hydroxide ("whiff test"), presence of "clue cells" (cells covered with bacteria) under the microscope;
• Nugent score: microscopic reading of the stained swab, which classifies the flora as normal, intermediate or vaginosis;
• molecular biology tests (PCR), increasingly used.

The swab also makes it possible to rule out thrush or trichomoniasis. It can be taken at the practice or at the laboratory.

What are the risk factors?

• a new partner or several partners;
• vaginal douching and repeated intimate washing;
• smoking;
• not using condoms;
• the copper intrauterine device, in some studies.

Bacterial vaginosis is linked to sexual activity. It is not classified as a sexually transmitted infection, but exchange of bacteria between partners plays a role, as confirmed by the StepUp trial.

How is it treated?

French and international guidelines recommend, for symptomatic vaginosis, one of the following treatments:

• oral metronidazole, most often for 7 days;
• metronidazole vaginal gel;
• clindamycin vaginal cream or oral clindamycin.

During treatment with metronidazole, it is advisable not to drink alcohol, because of the risk of feeling unwell (a disulfiram-like or "Antabuse" effect). Metronidazole can also leave a metallic taste in the mouth. Clindamycin cream can weaken latex condoms.

Why does vaginosis recur so often?

Recurrence is common: in the control group of the StepUp trial, where only the woman was treated, 63% of women had a recurrence within 12 weeks. Several mechanisms contribute:

• a bacterial biofilm on the vaginal wall, which antibiotics do not always eliminate;
• recolonisation by bacteria carried by the partner;
• lactobacilli that struggle to re-establish themselves.

Should the male partner be treated?

For a long time, the answer was no: earlier trials had not shown any benefit. The StepUp trial, published in 2025, changed the available evidence. It included monogamous heterosexual couples in which the woman had vaginosis:

• in the "partner treated" group, the woman received the usual treatment and the man took metronidazole 400 mg twice a day for 7 days, combined with clindamycin 2% cream applied to the skin of the penis, twice a day for 7 days;
• in the control group, only the woman was treated.

The trial was stopped earlier than planned, because the difference was clear: 35% recurrence at 12 weeks when the partner was treated, compared with 63% when he was not. Side effects in men were nausea, headaches and a metallic taste.

These results concern monogamous heterosexual couples. They are recent; the approach is discussed case by case at the consultation, particularly if you have recurrences.

What can be done about recurrent bacterial vaginosis?

In the event of frequent recurrences, several options can be discussed:

• confirming the diagnosis with a new swab;
• longer treatment or maintenance treatment with metronidazole vaginal gel, according to international guidelines;
• treatment of the partner, in the light of the StepUp trial;
• removing risk factors: stopping vaginal douching, stopping smoking, using condoms;
• vaginal probiotics based on lactobacilli: their value remains debated and the evidence is limited.

What about during pregnancy?

During pregnancy, vaginosis is associated with a slightly higher risk of premature birth and late miscarriage. The 2016 CNGOF guidelines on the prevention of spontaneous preterm birth state that:

• symptomatic vaginosis should be treated, with metronidazole or clindamycin, which can be used during pregnancy;
• systematic screening for vaginosis in pregnant women without symptoms and at low risk is not recommended to prevent preterm birth (grade A);
• in women with a previous preterm birth, systematic screening is not recommended either (grade C), except when that birth occurred in a context of infection, in which case early screening and treatment may be discussed.

The Cochrane review by Brocklehurst et al. (21 trials, 7,847 women) shows that antibiotics clear vaginosis during pregnancy and reduce the risk of late miscarriage, but do not reduce the overall risk of birth before 37 weeks. See "Pregnancy follow-up: appointments, scans and tests month by month".

Does vaginosis have other consequences?

Outside pregnancy, bacterial vaginosis is associated with a higher risk of infection after a gynaecological procedure (IUD insertion, hysteroscopy, termination of pregnancy) and of acquiring sexually transmitted infections.

Frequently asked questions

General information article: it does not replace a medical consultation. In an emergency, call 15 or 112 in France, or your local emergency number.