BV, or bacterial vaginosis, is the most common vaginal infection in women of reproductive age. It happens when the natural balance of bacteria in the vagina shifts, with helpful bacteria declining sharply and harmful bacteria multiplying to take their place. BV is not a sexually transmitted infection, though sexual activity is one of its strongest risk factors.
What Happens Inside the Vagina During BV
A healthy vagina is dominated by a group of protective bacteria called lactobacilli, which normally make up 70 to 90 percent of all vaginal bacteria in premenopausal women. These bacteria produce lactic acid, keeping the vaginal environment acidic (typically below a pH of 4.5). That acidity acts as a natural defense, suppressing the growth of other organisms.
In BV, lactobacilli are suddenly overtaken by a mix of anaerobic bacteria, organisms that thrive in low-oxygen environments. The most well-known of these is Gardnerella vaginalis, but the shift involves many species, including Atopobium vaginae, Prevotella, Mobiluncus, and Megasphaera, among others. As these bacteria multiply, vaginal pH rises above 4.5, creating an environment that makes it even easier for them to flourish and harder for lactobacilli to recover. This is why BV tends to persist or recur without treatment.
How BV Feels and What to Look For
Many women with BV have no symptoms at all. When symptoms do appear, the most recognizable one is a thin, off-white, gray, or greenish vaginal discharge with a noticeable fishy odor. The smell is often strongest after sex or during a period, because semen and menstrual blood are both alkaline, which triggers the release of the compounds responsible for the odor.
Unlike a yeast infection, BV rarely causes significant itching, redness, or swelling. The discharge itself tends to be thinner and more watery rather than thick or clumpy. This difference is one of the easiest ways to distinguish BV from a yeast infection at home, though the two can sometimes occur together.
What Increases Your Risk
BV rarely affects women who have never had sex. Beyond sexual activity, the CDC identifies three key behaviors that raise your risk: douching, not using condoms, and having new or multiple sexual partners. Douching is particularly harmful because it directly disrupts the vaginal bacterial balance that keeps BV at bay.
Other factors that can tip the balance include hormonal changes (like those from a new birth control method), smoking, and the use of scented soaps or body washes in the genital area. The vagina is self-cleaning, and introducing products that alter its chemistry can push conditions in the wrong direction.
How BV Is Diagnosed
Diagnosis usually involves a combination of a physical exam, a look at the discharge, and one or two simple tests. Clinicians typically check for at least three of the following four signs: vaginal pH above 4.5, the presence of “clue cells” (vaginal cells coated in bacteria visible under a microscope), a fishy odor when a chemical solution is added to a sample of discharge, and the characteristic thin, homogeneous discharge.
In some settings, a lab-based scoring system is used instead. A technician examines a vaginal swab under a microscope and scores the ratio of lactobacilli to BV-associated bacteria on a scale of 0 to 10. A score of 7 or higher confirms BV. This method is considered the gold standard for research but isn’t always used in routine clinical visits, where the simpler criteria are usually enough.
Treatment and What to Expect
BV is treated with antibiotics, either taken orally or applied as a vaginal cream. The standard oral treatment is a seven-day course of metronidazole, taken twice daily. Alternatively, a clindamycin vaginal cream applied at bedtime for seven days works just as well for many women. Shorter courses using tinidazole (a related antibiotic) are also available.
Most women notice improvement within two to three days of starting treatment, with the odor resolving first and the discharge gradually returning to normal. Alcohol should be avoided during metronidazole treatment and for at least 24 hours afterward, as the combination can cause nausea and vomiting.
The biggest challenge with BV is recurrence. Within 6 to 12 months of finishing antibiotic therapy, 50 to 80 percent of women will experience a return of symptoms. This high recurrence rate is one of the most frustrating aspects of the condition, and it’s an area where prevention habits become especially important.
Why BV Keeps Coming Back
Researchers still don’t fully understand why BV recurs so frequently. One leading theory is that the anaerobic bacteria form a protective layer, called a biofilm, on the vaginal wall that antibiotics can reduce but not fully eliminate. Once treatment stops, the surviving bacteria can repopulate quickly. Another possibility is reintroduction of bacteria through sexual contact, since the organisms associated with BV can be carried by partners.
For women dealing with frequent recurrence, strategies that help include using condoms consistently, avoiding douching entirely, and in some cases, extended or suppressive antibiotic regimens prescribed by a healthcare provider. Probiotic supplements containing lactobacillus strains are widely marketed for vaginal health, but evidence for their effectiveness in preventing BV recurrence remains mixed.
BV During Pregnancy
BV during pregnancy deserves special attention because it has been linked to a higher risk of preterm delivery, meaning birth before 37 weeks. Preterm birth can lead to serious complications for the baby, including breathing problems and developmental issues. The relationship between BV and preterm delivery is well-documented, though it is not yet clear whether BV directly causes early labor or is a marker of other factors that contribute to it.
Pregnant women who develop symptoms of BV are typically treated with the same antibiotics used outside of pregnancy. If you’re pregnant and notice a change in discharge or an unusual odor, it’s worth bringing up at your next prenatal visit rather than assuming it will resolve on its own.
BV and Susceptibility to Other Infections
The vaginal environment created by BV, with reduced acidity and depleted lactobacilli, makes it easier for sexually transmitted infections to take hold. Women with BV have a higher risk of acquiring HIV, herpes, chlamydia, and gonorrhea compared to women with a healthy vaginal balance. The protective acid barrier that normally helps block these pathogens is weakened, and the inflammation caused by BV may make vaginal tissue more vulnerable to infection.
This is one of the reasons that treating BV matters even when symptoms are mild or absent. Beyond the immediate discomfort, untreated BV can leave you more exposed to infections with longer-term consequences.