Bacterial vaginosis is not officially classified as a sexually transmitted infection, but sex is one of the strongest risk factors for developing it. The distinction matters: BV can occur in people who have never had sex, but sexual activity significantly increases the likelihood of getting it and makes it more likely to come back. Between 23% and 29% of women of reproductive age worldwide have BV at any given time, making it the most common vaginal condition in that age group.
Why BV Isn’t Classified as an STI
A true STI is caused by a specific pathogen passed from one person to another during sex. BV doesn’t work that way. It’s a shift in the vaginal microbiome, where the protective bacteria that keep the vagina slightly acidic get crowded out by a mix of other bacteria. No single organism causes it, and it can develop after anything that disrupts vaginal pH, including douching, certain soaps, or hormonal changes.
That said, the line between “sexually transmitted” and “sexually associated” is blurring. Researchers increasingly treat BV as a condition with a strong sexual transmission component, even if it doesn’t fit the classic STI model.
How Sex Contributes to BV
Sex affects vaginal bacterial balance in several ways. Semen is alkaline, which temporarily raises vaginal pH and creates conditions where BV-associated bacteria can thrive. Lubricants can have a similar effect. A new sexual partner introduces unfamiliar bacteria, and having multiple partners compounds that exposure. Inconsistent condom use also correlates with higher BV risk, while consistent condom use is associated with lower risk.
The CDC lists the major sexual risk factors for BV as having multiple male sex partners, female sex partners, a new sex partner, and not using condoms. Women who have sex with women also have elevated BV rates, likely because the bacteria transfer between partners through shared vaginal contact.
BV-Linked Bacteria Live in Male Partners Too
One of the most compelling pieces of evidence for sexual transmission is that the bacteria associated with BV are commonly found in the male urogenital tract. A study of 144 men presenting with urinary symptoms or concerns after unprotected sex found that Gardnerella vaginalis, the bacterium most closely linked to BV, was the single most frequently detected organism, present in about 16% of samples. In 35% of those positive cases, it was the only bacterium found.
Research on healthy men without symptoms has found similar results. Gardnerella appears as one of the predominant bacteria in first-void urine samples from both healthy males and those with unexplained urethritis. The biofilm form of Gardnerella, which is the sticky, antibiotic-resistant coating it builds inside the vagina, has been specifically suggested to be sexually transmissible. This means men can carry and pass along the exact bacterial communities that trigger BV, often without knowing it.
Treating Male Partners Reduces Recurrence
BV is notorious for coming back. Up to half of women who get treated will have a recurrence within months. For years, guidelines did not recommend treating male sexual partners because BV wasn’t considered an STI. A landmark study published in the New England Journal of Medicine in March 2025 challenged that approach directly.
The study enrolled women in monogamous heterosexual relationships who were diagnosed with and treated for BV. Their male partners were randomly assigned to either receive antibiotics (oral medication plus a topical cream applied to the penis) or no treatment at all. Among the 81 couples where the man was treated, BV returned 35% of the time. In the 81 couples where only the woman was treated, the recurrence rate was 63%. The difference was so significant that the study was stopped early because the untreated group was doing considerably worse.
This nearly doubles the chance of staying BV-free simply by treating the male partner, which is strong evidence that reinfection from a sexual partner plays a major role in why BV keeps coming back. Treated men did report side effects, including headache, nausea, and a metallic taste, in about 46% of cases.
Other Risk Factors Beyond Sex
Sex isn’t the only trigger. Douching is one of the best-documented non-sexual risk factors because it directly washes away protective bacteria and disrupts vaginal pH. Hormonal shifts during the menstrual cycle can also play a role, as vaginal pH naturally rises during menstruation. Antibiotics taken for other infections sometimes kill off protective vaginal bacteria as a side effect, opening the door for BV-associated organisms to take over.
People who have never had sex can and do develop BV, which is the core reason it remains classified as sexually associated rather than sexually transmitted. But the pattern is clear: sexual activity, especially with new or multiple partners and without condoms, is the single biggest predictor of who develops BV and who develops it repeatedly.
Reducing Your Risk
Consistent condom use is one of the most effective ways to lower your BV risk during sex. Condoms prevent semen from raising vaginal pH and reduce the exchange of bacteria between partners. Avoiding douching is equally important, since it strips away the very bacteria your vagina needs to protect itself.
If you’re dealing with recurrent BV in a stable relationship, the 2025 partner treatment data is worth discussing with your healthcare provider. Current guidelines haven’t yet been updated to recommend routine partner treatment, but the evidence is shifting. Treating both partners at the same time may be the most effective strategy for breaking the cycle of reinfection, particularly if BV keeps returning after your own treatment.