Bacterial vaginosis (BV) develops when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria called Lactobacillus dominate the vaginal environment, producing acids and antimicrobial compounds that keep harmful bacteria in check. When Lactobacillus levels drop, anaerobic bacteria multiply rapidly to fill the gap. An estimated 23 to 29% of women of reproductive age worldwide have BV at any given time, making it the most common vaginal condition in that age group.
BV is not classified as a sexually transmitted infection, though sexual activity is one of the strongest risk factors. The full cause of the bacterial shift isn’t completely understood, but several well-documented triggers explain how it happens.
What Happens Inside the Vagina
A healthy vagina maintains a slightly acidic pH, roughly between 3.8 and 4.5. Lactobacillus bacteria are responsible for this acidity. They produce lactic acid and hydrogen peroxide, creating an environment that’s hostile to most harmful microbes. When something disrupts these Lactobacillus populations, the pH rises and conditions become favorable for anaerobic bacteria to flourish.
The bacterium most commonly found in BV is Gardnerella vaginalis. Once it gains a foothold, it forms a thick, sticky biofilm on the vaginal lining. This biofilm acts like a protective shield, sheltering Gardnerella and other harmful bacteria from both the body’s immune defenses and antibiotic treatment. The biofilm is a major reason BV is so difficult to fully clear and so prone to coming back. Research published in Antimicrobial Agents and Chemotherapy found that roughly 80% of women who are treated for BV will have another episode within a year, largely because antibiotics struggle to penetrate and destroy the biofilm completely.
How Sexual Activity Plays a Role
Sex is one of the most significant triggers for BV, even though BV itself isn’t an STI. Semen is alkaline, and exposure to it raises vaginal pH, shifting conditions away from the acidic environment Lactobacillus needs to thrive. Research in The Journal of Sexual Medicine found that factors in semen directly promote the growth of pathogenic bacteria while simultaneously inhibiting Lactobacillus. This means unprotected sex can create a temporary window where harmful bacteria gain an advantage.
Having a new sexual partner or multiple partners increases BV risk further, likely because each partner introduces different microbial communities. Women who have sex with women can also pass BV-associated bacteria between partners. Condom use appears to reduce risk by preventing direct contact with semen and limiting bacterial exchange.
Douching and Vaginal Products
Douching is one of the most well-documented behavioral risk factors. A study in American Family Physician found that women who douched at least once a month had a 1.4 times higher risk of developing BV. Women who had douched within the past week had 2.1 times the risk. The mechanism is straightforward: douching washes away hydrogen peroxide and Lactobacillus, stripping the vagina of its primary defense system. Without those protective bacteria, anaerobic organisms overgrow quickly.
Scented soaps, bubble baths, vaginal deodorants, and scented tampons or pads can cause similar disruption on a smaller scale. The vagina is self-cleaning, and introducing chemicals interferes with the bacterial ecosystem that keeps it healthy.
Smoking and Hormonal Factors
Cigarette smoking increases BV risk through a surprisingly direct route. Smoking has an anti-estrogenic effect, which matters because estrogen helps maintain the vaginal lining and supports Lactobacillus colonization. On top of that, a chemical byproduct of cigarette smoke called BPDE has been detected in the vaginal secretions of smokers. This compound triggers the destruction of Lactobacillus bacteria by activating dormant viruses within them, essentially killing the very bacteria that protect against BV.
Hormonal shifts from other causes can also play a role. Any drop in estrogen, whether from certain contraceptives, breastfeeding, or perimenopause, can thin the vaginal lining and reduce the nutrients Lactobacillus relies on, creating an opening for harmful bacteria.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. Even after a full course of antibiotics clears symptoms, the Gardnerella biofilm often survives. The biofilm is encased in a matrix of protective substances that antibiotics can’t easily penetrate. Once treatment stops, the bacteria sheltered within the biofilm begin multiplying again, and symptoms return within three to twelve months for the majority of women.
Ongoing exposure to the same triggers, like unprotected sex or douching, compounds the problem. Each new disruption to the vaginal pH gives the biofilm another opportunity to regrow. Gardnerella strains are also showing increasing resistance to the standard antibiotics used to treat BV, making complete eradication even harder.
What BV Feels Like
The hallmark symptom is a thin, grayish-white vaginal discharge with a distinct fishy odor, often more noticeable after sex. Some women experience mild itching or burning during urination. But roughly half of women with BV have no symptoms at all and only discover it during a routine exam. The absence of symptoms doesn’t mean the condition is harmless. Untreated BV increases susceptibility to sexually transmitted infections and, during pregnancy, raises the risk of preterm delivery.
Reducing Your Risk
The most effective prevention strategies target the triggers that disrupt Lactobacillus. Avoiding douching is the single most impactful change, since it eliminates a major source of bacterial disruption. Using condoms reduces exposure to semen’s pH-altering effects. Choosing unscented hygiene products and avoiding internal washes keeps the vaginal ecosystem intact.
Quitting smoking removes the chemical assault on Lactobacillus. Wearing breathable cotton underwear and changing out of wet swimwear or workout clothes promptly helps maintain conditions that favor beneficial bacteria. Some evidence supports probiotic use to help restore Lactobacillus populations, though the strength of that evidence varies depending on the strain and delivery method.