Bacterial vaginosis (BV) develops when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria keep the vaginal environment slightly acidic, which prevents harmful organisms from growing. When those protective bacteria decline and other types take over, BV is the result. It’s the most common vaginal condition in people of reproductive age, and despite how frequently it occurs, the exact trigger isn’t fully understood.
What Happens Inside the Vagina
A healthy vagina is dominated by a single species of protective bacteria that produces lactic acid, keeping the environment at a low pH (below 4.5). This acidity acts as a natural defense system, making it difficult for other organisms to establish themselves. The most protective species is called Lactobacillus crispatus, though a few related species also do the job well.
BV occurs when these protective bacteria are displaced by a diverse mix of other bacteria that thrive in less acidic conditions. As these organisms multiply, they raise the vaginal pH above 4.5, produce compounds that cause a fishy odor, and create the thin, grayish-white discharge that characterizes the condition. This isn’t a single “infection” caused by one invading germ. It’s a community-wide shift in the bacterial ecosystem.
One detail that helps explain why BV is so common and so stubborn: not all protective vaginal bacteria are equally protective. A species called Lactobacillus iners is frequently found in the vagina, but communities dominated by it carry a higher risk of transitioning to BV compared to communities dominated by other species. This matters because standard antibiotic treatment tends to shift the vaginal environment toward L. iners dominance rather than the more protective species, which may help explain why recurrence rates are so high.
Sexual Activity and BV
BV is not classified as a sexually transmitted infection, but sexual activity is the single strongest association. The condition rarely affects people who have never had sex. New or multiple sexual partners increase the risk, as does not using condoms. The CDC states plainly that researchers still don’t know exactly how sex contributes to BV, only that it clearly does.
The leading theory is that sexual contact introduces new bacteria or disrupts the existing bacterial balance. This can happen with male or female partners. For women who have sex with women, BV-associated bacteria can be shared between partners, and concordant BV (both partners affected) is well documented. Because BV isn’t caused by a single pathogen, treating one partner with antibiotics doesn’t reliably prevent the other from developing it, which is one reason it behaves differently from a classic STI.
Douching and Hygiene Products
Douching is one of the most well-established risk factors. Women who douche once a week are five times more likely to develop BV than women who don’t douche at all. The reason is straightforward: douching washes away the protective bacteria and disrupts the natural acidity that keeps harmful organisms in check. Even a single rinse can shift the balance enough to allow other bacteria to gain a foothold.
Scented tampons, pads, vaginal powders, and sprays pose a similar risk. These products can irritate vaginal tissue and alter its chemistry. The vagina is self-cleaning, and introducing products inside it tends to do more harm than good. External washing with mild, unscented soap is all that’s needed.
Smoking, IUDs, and Other Risk Factors
Cigarette smoking has a dose-dependent relationship with BV, meaning the more you smoke, the higher your risk. Research has found that smoking alters the metabolic environment of the vagina in ways that may make it less hospitable to protective bacteria, though the exact mechanism is still being studied.
Intrauterine devices (IUDs) are also associated with higher BV rates. One study found BV in 47% of IUD users compared to 30% of non-users, an association that held even after accounting for other risk factors like age and douching history. The string of the IUD may provide a surface for bacteria to cling to, or the device itself may subtly alter the vaginal environment.
Other factors that can tip the bacterial balance include antibiotic use (which can wipe out protective bacteria along with whatever infection is being treated), hormonal changes, and stress. Anything that reduces the population of acid-producing bacteria or raises vaginal pH creates an opening for BV-associated organisms.
How BV Is Identified
BV typically shows up as a thin, milklike discharge with a noticeable fishy smell, especially after sex. Some people have no symptoms at all and only discover it during a routine exam. Clinicians diagnose it by checking for at least three of four signs: the characteristic discharge, a vaginal pH above 4.5, a fishy odor when a chemical solution is applied to a sample, and the presence of “clue cells” (vaginal cells coated with bacteria) under a microscope.
If you’ve had BV before, the symptoms are usually recognizable. But similar symptoms can come from yeast infections or other conditions, so getting it confirmed rather than self-treating is worthwhile, especially the first time.
Why BV Keeps Coming Back
Recurrence is the defining frustration of BV. Within 6 to 12 months of finishing antibiotic treatment, 50% to 80% of women will experience it again. This extraordinarily high recurrence rate suggests that antibiotics clear the overgrown bacteria but don’t fully restore the protective community that prevents the condition in the first place.
Part of the problem is that treatment tends to leave behind L. iners, the less protective species, rather than re-establishing the L. crispatus communities associated with long-term vaginal health. The harmful bacteria may also persist in biofilms (thin layers of organisms that adhere to vaginal tissue and resist antibiotics), giving them a head start once treatment ends. Ongoing exposure to the same risk factors, whether a sexual partner carrying BV-associated bacteria, continued douching, or smoking, makes recurrence even more likely.
Reducing recurrence often means addressing the underlying risk factors alongside treatment. Stopping douching, using condoms consistently, and quitting smoking can all shift the odds. Some clinicians recommend extended or suppressive antibiotic regimens for people with frequent recurrences, though no single approach has solved the problem reliably.