Bacterial vaginosis (BV) happens when the balance of bacteria inside the vagina shifts, with protective bacteria declining and harmful bacteria multiplying to take their place. It affects roughly 23 to 29% of women of reproductive age worldwide, making it the most common vaginal infection. The exact cause isn’t fully understood, but several well-established triggers can set it off.
What Happens Inside the Vagina
A healthy vagina is dominated by a type of protective bacteria called lactobacilli. These bacteria produce antimicrobial compounds that keep the vaginal environment acidic, typically at a pH below 4.5. That acidity acts like a natural defense system, suppressing the growth of harmful microorganisms.
BV develops when lactobacilli sharply decline, and anaerobic bacteria (the kind that thrive without oxygen) multiply in their place. The most commonly identified organism in BV is Gardnerella vaginalis, though the infection usually involves a mix of several bacterial species. As these bacteria take over, vaginal pH rises above 4.5, creating an environment where they grow even faster. This is why BV can feel like it comes on quickly once the shift begins.
Sexual Activity Is the Strongest Risk Factor
BV is not classified as a sexually transmitted infection, but sexual activity is closely linked to it. BV rarely affects people who have never had sex. Researchers still don’t fully understand the mechanism connecting sex to BV, but the pattern is consistent: having new or multiple sexual partners and not using condoms both increase the risk. Sex can introduce new bacteria into the vaginal environment or physically disrupt the existing bacterial community, giving anaerobic species an opening to outcompete lactobacilli.
Partner treatment is now getting more attention. The American College of Obstetricians and Gynecologists recently recommended treating sexual partners at the same time for people with recurrent BV, a recognition that bacteria can be shared back and forth between partners.
Douching and Hygiene Products
Douching is one of the most well-documented non-sexual triggers for BV. Flushing the vagina with water, vinegar, or commercial douching products washes away the protective lactobacilli and disrupts the natural pH. The CDC lists douching alongside unprotected sex and new partners as a direct risk factor.
Other vaginal hygiene products can cause similar problems. Harsh soaps, scented washes, and products containing synthetic fragrances, parabens, or other chemical preservatives can irritate the vaginal lining and throw off pH balance. Some products contain numbing agents like benzocaine that mask irritation rather than preventing it. The vagina is self-cleaning, so these products solve a problem that doesn’t exist while creating a real one.
Antibiotics Can Backfire
This is one of BV’s frustrating paradoxes: antibiotics are the standard treatment, but they can also be a trigger. Antibiotics don’t just target the infection you’re treating. They kill bacteria broadly, including the protective lactobacilli in your vagina. If you take antibiotics for a urinary tract infection, a sinus infection, or anything else, the resulting disruption to your vaginal microbiome can create conditions for BV to develop. This is one reason BV sometimes seems to appear “out of nowhere” during or after a course of antibiotics for an unrelated issue.
Smoking
Cigarette smoking raises BV risk through a less obvious pathway. Studies have found that smoking depletes vaginal lactobacilli, increases vaginal pH, and promotes the growth of the anaerobic bacteria associated with BV. The effect appears to be chemical rather than behavioral. Compounds absorbed from cigarette smoke reach vaginal tissue through the bloodstream and alter the local environment in ways that favor harmful bacteria.
Hormonal Changes
Estrogen plays a key role in supporting lactobacilli populations. When estrogen levels drop, so do levels of protective bacteria. This makes certain life stages higher risk for BV. During menopause, declining estrogen is a well-recognized trigger. Pregnancy also increases BV risk, though for a different reason: the rapid hormonal fluctuations of pregnancy can destabilize the vaginal microbiome even though estrogen levels are high overall. Hormonal shifts during the menstrual cycle can also create temporary windows of vulnerability, which is why some people notice BV symptoms appearing at predictable points in their cycle.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. Up to 66% of people who are treated for BV experience it again within a year. Several factors explain this. The antibiotics used to treat BV kill the overgrown anaerobic bacteria but don’t actively rebuild the lactobacilli population, so the vagina can remain vulnerable even after symptoms resolve. If the original trigger is still present, whether that’s an ongoing sexual partnership sharing bacteria, continued douching, or smoking, the conditions that caused the first episode simply cause another.
Some people also appear to have vaginal microbiomes that are naturally less stable, with lower baseline lactobacilli levels that make recolonization slower after treatment. This isn’t something you did wrong. It’s a biological variation that makes management more of an ongoing process than a one-time fix.
Reducing Your Risk
You can’t control every factor, but the modifiable ones make a real difference:
- Skip douching entirely. There is no medical benefit, and the harm is well documented.
- Avoid scented vaginal products. This includes scented tampons, wipes, sprays, and soaps with synthetic fragrances. Plain warm water is sufficient for external cleaning.
- Use condoms with new partners. Barrier methods reduce the introduction of bacteria that can disrupt vaginal flora.
- Quit smoking. The vaginal microbiome is one of many systems that benefit.
- Be aware of antibiotic side effects. If you’re prone to BV and need antibiotics for another condition, let your provider know so they can monitor or plan accordingly.
BV is common, treatable, and not a reflection of poor hygiene. In most cases, the opposite is true: doing less to the vagina and letting its natural ecosystem do its job is the most effective prevention strategy.