Bacterial vaginosis (BV) happens when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria called lactobacilli make up more than 70% of the vaginal microbiome and produce lactic acid that keeps the pH at 4.5 or below. When those protective bacteria decline, other organisms that thrive in less acidic environments multiply rapidly, and the result is BV. It is the most common vaginal condition in women of reproductive age.
BV is not something you “catch” the way you catch a cold. You cannot get it from toilet seats, bedding, or swimming pools. It develops from changes inside your own body, though several external factors can set those changes in motion.
What Happens Inside the Vagina
Estrogen drives the process that keeps the vagina healthy. It thickens the vaginal lining and signals cells to produce glycogen, a stored sugar. Lactobacilli feed on that glycogen (with help from a natural enzyme that breaks it down into usable pieces) and convert it into lactic acid. That acid environment is hostile to most harmful bacteria.
When something disrupts this cycle, lactobacilli lose their foothold. The pH climbs above 4.5, and anaerobic bacteria that were previously kept in check begin to flourish. The species most commonly involved include Gardnerella, Prevotella, Mobiluncus, and Sneathia. Once these organisms reach high enough numbers, they form a biofilm on the vaginal walls, which is part of why BV can be so stubborn to treat.
Sexual Activity and New Partners
Sexual activity is one of the strongest risk factors for developing BV, though researchers do not classify it as a sexually transmitted infection. The connection is real but indirect: new sexual partners introduce unfamiliar bacteria, and the exchange of body fluids can alter vaginal pH. Women who have sex with women can pass BV-associated bacteria between partners, which is one reason the condition clusters in some couples.
Having multiple partners or a new partner increases the likelihood of a disruption to the vaginal microbiome. Condom use appears to lower the risk, likely because it reduces direct exposure to semen, which has an alkaline pH around 7.2 to 8.0 and can temporarily raise vaginal pH after intercourse.
Douching and Hygiene Products
Douching is one of the clearest self-inflicted triggers for BV. Women who douche at least once a month have a 1.4-fold higher risk of developing BV or an intermediate imbalance in vaginal bacteria. For women who douched within the week before being tested, the risk jumped to 2.1 times higher. Douching disrupts the hydrogen peroxide and lactic acid defenses that lactobacilli provide, giving anaerobic bacteria room to take over.
Other products can cause similar problems. Bar soaps, bubble baths, scented shower gels, vaginal deodorants, and even some over-the-counter creams (including herbal products like tea tree oil or aloe vera) can irritate vaginal tissue and shift pH toward alkaline. The Royal College of Obstetricians and Gynaecologists recommends avoiding soap, scrubs, baby wipes, and deodorant on the vulva entirely. Cleaning with warm water alone, or with a gentle wash specifically formulated to match vulvar pH, is sufficient.
Conventional panty liners with non-breathable backing can also change the local environment by trapping moisture and heat, raising both temperature and pH at the skin surface.
Smoking and Vaginal Health
Cigarette smoking has a surprisingly strong link to BV. In one study, women whose vaginal microbiomes lacked significant lactobacilli were 25 times more likely to be current smokers than women with healthy, lactobacillus-dominant communities.
The mechanism is direct, not just statistical. Nicotine’s main breakdown product, cotinine, concentrates in cervical mucus, meaning smoking physically alters the chemical environment of the vagina. In smokers with disrupted microbiomes, researchers found substantially higher levels of compounds called biogenic amines. These amines raise pH (making the environment less acidic and less protective) and contribute to the fishy odor associated with BV. Nicotine itself is a weak base, which means its presence in vaginal tissue works against the acidic conditions lactobacilli need to thrive.
Hormonal Changes
Because estrogen is the upstream driver of glycogen production, anything that lowers estrogen can reduce lactobacilli and raise BV risk. This includes the postpartum period, breastfeeding, perimenopause, and menopause. Some hormonal contraceptives can also influence the microbiome, though the relationship varies by type. Menstrual blood, which has a neutral to slightly alkaline pH, temporarily raises vaginal pH each month, which is why some women notice symptoms flare around their period.
What BV Feels Like
About half of women with BV have no symptoms at all. When symptoms do appear, the most recognizable one is a thin, grayish-white discharge with a fishy smell. The odor often becomes stronger after sex or during menstruation. The discharge tends to be uniform and milky rather than clumpy, which helps distinguish it from a yeast infection. Itching or burning is possible but less common than with yeast.
Doctors typically diagnose BV using a combination of signs: the characteristic discharge, 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 in bacteria) under a microscope. At least three of these four markers need to be present.
Why BV Keeps Coming Back
One of the most frustrating aspects of BV is its recurrence rate. Within 6 to 12 months of finishing antibiotic treatment, 50% to 80% of women experience a return of symptoms. Antibiotics kill the overgrown anaerobic bacteria effectively, but they do not rebuild the lactobacillus population. Once treatment ends, if lactobacilli have not re-established dominance, the same cycle of pH rise and anaerobic overgrowth begins again.
The biofilm that BV-associated bacteria form on vaginal walls is part of the problem. It can shield bacteria from antibiotics, leaving behind a reservoir that repopulates once treatment stops. Ongoing exposure to the same triggers, whether that is douching, smoking, or reintroduction of bacteria from a sexual partner, also drives recurrence.
Lowering Your Risk
No single step guarantees prevention, but reducing known triggers makes a meaningful difference. Stopping douching removes one of the most well-documented risk factors. Switching from scented soaps and body washes to plain water for vulvar cleaning eliminates unnecessary pH disruption. Quitting smoking addresses a major and often overlooked contributor. Using condoms with new partners reduces bacterial exchange.
Probiotics containing Lactobacillus crispatus strains are an active area of interest. In a Phase 2b clinical trial, women who used a vaginal L. crispatus product after antibiotic treatment had significantly lower rates of BV recurrence at both 12 and 24 weeks compared to placebo. About 79% of treated women showed colonization of the probiotic strain during the treatment period, though that dropped to 48% after the product was discontinued. These products are not yet widely available as approved treatments, but the results suggest that restoring lactobacilli directly, rather than relying on antibiotics alone, may eventually become a standard approach to breaking the recurrence cycle.
Wearing breathable cotton underwear and avoiding tight synthetic clothing helps maintain a cooler, drier environment that favors healthy bacteria. Choosing panty liners with breathable backing, if you use them at all, prevents the moisture and heat buildup that can nudge pH in the wrong direction.