Bacterial vaginosis (BV) happens when the natural balance of bacteria inside the vagina shifts. Normally, beneficial bacteria keep the vaginal environment slightly acidic (a pH below 4.5), which prevents harmful microbes from thriving. When those protective bacteria drop in number and other organisms multiply in their place, the result is BV. It’s the most common vaginal condition in women of reproductive age, and despite how frequently it occurs, the exact trigger isn’t fully understood.
What Actually Happens Inside the Vagina
A healthy vagina is dominated by protective bacteria that produce lactic acid, keeping the pH low and the environment inhospitable to most harmful organisms. One species in particular is especially effective: it directly reduces the damage caused by the primary bacterium involved in BV by suppressing the toxins it releases.
When BV develops, those protective bacteria are suddenly replaced by a mix of anaerobic organisms, meaning bacteria that thrive without oxygen. This shift raises vaginal pH above 4.5, creating an environment where these organisms can flourish even further. The overgrowth isn’t caused by a single “bad” germ. It’s a whole community of different bacteria taking over at once, which is part of why BV is so tricky to pin down and so prone to coming back after treatment.
The Role of Sexual Activity
BV is not classified as a sexually transmitted infection, but sex is closely linked to it. The CDC states that BV rarely affects people who have never had sex, and it most often occurs in those who are sexually active. New or multiple sexual partners and not using condoms both increase the risk. The introduction of new bacteria, changes in pH from semen, and disruption of the vaginal environment during intercourse all likely play a role, though researchers still don’t fully understand the mechanism.
BV can spread between female sexual partners. Male partners, however, do not need treatment, and there’s no evidence that treating a male partner reduces recurrence. This is one of the clearest signs that BV isn’t a straightforward infection passed from one person to another. It’s more accurately described as a disruption of an ecosystem.
Douching and Hygiene Products
Douching is one of the most well-documented risk factors for BV. Women who douche at least once a month have about 1.4 times the risk of developing BV compared to those who don’t. That risk jumps to 2.1 times for women who douched within the past week. The brand of douche doesn’t matter; both major commercial brands studied carried similar risk levels.
The vagina is self-cleaning, and introducing water, vinegar, or scented solutions disrupts the protective bacterial community. Scented soaps, sprays, and washes applied inside or around the vagina can have a similar effect by altering pH and killing off beneficial bacteria. Washing the external vulva with plain water or a gentle, unscented soap is all that’s needed.
Hormonal Changes
Hormone fluctuations throughout the menstrual cycle influence BV risk. BV is more commonly observed during the first half of the cycle, before ovulation, when estrogen levels are still rising. Hormonal shifts affect the vaginal lining and the mucus it produces, which in turn changes the surface that bacteria attach to and grow on. This is why some people notice BV symptoms appearing at roughly the same point in their cycle each month.
Pregnancy, menopause, and hormonal contraceptives can also shift the vaginal environment in ways that make BV more or less likely, though the relationship varies from person to person.
Other Risk Factors
- Smoking. Tobacco use is consistently associated with higher BV rates, likely because compounds from cigarette smoke concentrate in cervical mucus and may suppress protective bacteria.
- IUDs. Some people develop BV after having an intrauterine device inserted, possibly due to changes in the vaginal environment during and after the procedure.
- Antibiotics for other conditions. Broad-spectrum antibiotics can wipe out protective vaginal bacteria along with whatever infection they’re targeting, leaving the door open for anaerobic overgrowth.
How BV Is Recognized
The classic symptoms are a thin, grayish-white discharge with a noticeable fishy smell, especially after sex. Some people also experience mild itching or burning during urination, though many have no symptoms at all. Up to half of BV cases are asymptomatic.
A healthcare provider can confirm BV using a few straightforward checks: examining the discharge for a milklike consistency, testing vaginal pH (above 4.5 suggests BV), looking at a sample under a microscope for cells coated in bacteria, and checking whether the discharge produces a fishy odor when exposed to a chemical solution. Meeting at least three of those four criteria confirms the diagnosis.
Why Untreated BV Matters
BV sometimes clears on its own, but leaving it untreated carries real risks. The disrupted vaginal environment makes it easier for sexually transmitted infections, including HIV, chlamydia, and gonorrhea, to take hold. During pregnancy, untreated BV is linked to preterm birth and low birth weight. It can also increase the risk of infection after gynecological procedures.
Treatment typically involves a short course of antibiotics, either taken orally or applied as a vaginal gel. The bigger challenge is recurrence. Roughly half of people treated for BV will have it return within 12 months, often because the underlying bacterial imbalance was never fully corrected.
Lowering Your Risk
Because BV stems from a disrupted bacterial balance rather than a single invading germ, prevention focuses on protecting the vaginal environment. Avoiding douching is the single most impactful step. Using condoms with new partners reduces exposure to bacteria and semen that can shift pH. Choosing unscented products for anything that contacts the vulva or vagina helps preserve the natural microbial community.
Cotton underwear and avoiding prolonged time in wet clothing (like swimsuits) keep the area dry, which favors protective bacteria over anaerobes. Some research suggests that probiotics containing specific protective bacterial strains may help, but the evidence isn’t strong enough yet to make broad recommendations. The most reliable approach remains minimizing the things known to disrupt vaginal flora: douching, scented products, and unprotected sex with new partners.