Bacterial vaginosis (BV) happens when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment and keep it acidic, with a pH between 4.0 and 4.5. When those protective bacteria decline, a mix of other organisms that were previously present in small numbers multiply rapidly and take over, pushing the pH above 4.5 and triggering the symptoms associated with BV.
What Changes Inside the Vagina
A healthy vagina is not sterile. It contains a community of microorganisms, and lactobacilli are the most important residents. These bacteria produce lactic acid and hydrogen peroxide, which maintain the acidic environment that keeps harmful organisms in check. BV develops when something disrupts this system, allowing species like Gardnerella vaginalis, Atopobium vaginae, Prevotella, Mobiluncus, and others to proliferate dramatically.
Gardnerella vaginalis plays a particularly central role. It has a higher ability to adhere to vaginal tissue and cause cell damage than most other BV-associated species. It also forms a sticky, structured layer called a biofilm along the vaginal lining. This biofilm creates a low-oxygen pocket where anaerobic bacteria thrive. Other organisms embed themselves within it, essentially using it as a shelter to grow and resist treatment. This biofilm is a major reason BV can be stubborn and recurrent.
As these anaerobic bacteria multiply, they produce amines, which are the compounds responsible for the characteristic fishy odor. They also create the thin, grayish-white discharge that coats the vaginal walls. The shift from an acidic to a more alkaline environment (pH above 4.5) is both a consequence and a perpetuating factor: once the pH rises, conditions favor the very bacteria causing the problem.
What Triggers the Shift
No single cause explains every case of BV, but several well-established factors make the disruption more likely.
Sexual activity is one of the strongest. BV is not classified as a sexually transmitted infection because no single organism causes it, and it can occur in people who have never had sex. But the connection to sexual behavior is hard to ignore. Research dating back to 1955 showed that vaginal material from women with BV could disrupt normal flora in other women, and that condom use appeared protective. Since then, studies have consistently found that BV’s risk profile mirrors that of STIs: new or multiple partners, inconsistent condom use, and female sex partners all increase risk. Having a regular sex partner throughout the course of treatment is also associated with higher recurrence.
Douching is another significant trigger. Introducing water, vinegar, or commercial products into the vagina washes away lactobacilli and disrupts the microbial community, creating an opening for harmful bacteria to colonize. The vagina is self-cleaning, and douching works against that process.
Other contributing factors include recent antibiotic use (which can kill off protective lactobacilli along with targeted bacteria), certain forms of contraception, and hormonal changes. Smoking has also been linked to BV, though the mechanism is less clear.
What BV Feels Like
Many people with BV have no symptoms at all. When symptoms do appear, they typically include a thin, milklike vaginal discharge, a noticeable fishy smell (often stronger after sex), and mild irritation. BV does not usually cause the intense itching or thick discharge associated with yeast infections, which is one way to tell them apart, though overlap is common enough that self-diagnosis is unreliable.
Clinicians confirm BV using a set of criteria that includes checking whether the vaginal pH is above 4.5, looking for a fishy odor when a chemical solution is applied to a discharge sample, examining the discharge under a microscope for characteristic “clue cells” (vaginal cells coated in bacteria), and assessing the consistency of the discharge itself. Meeting three of these four criteria confirms the diagnosis.
Why BV Keeps Coming Back
One of the most frustrating aspects of BV is its recurrence rate. In one study following women after standard antibiotic treatment, 58% had a recurrence within 12 months, and 69% had abnormal vaginal flora return in the same period. The biofilm that Gardnerella vaginalis builds on the vaginal lining is a likely culprit: antibiotics can reduce symptoms without fully eradicating the biofilm, allowing the bacterial community to re-establish itself.
Factors associated with recurrence include a history of prior BV episodes, having a regular sexual partner during treatment (suggesting possible reintroduction of disrupting bacteria), and having female sexual partners. Hormonal contraception, interestingly, appears to have a protective effect against recurrence, possibly because it stabilizes the hormonal environment that supports lactobacilli.
Restoring and Protecting Vaginal Flora
Because BV fundamentally involves the loss of protective lactobacilli, restoring those populations is a logical approach to prevention. Probiotics containing specific strains have shown measurable benefit. In clinical trials, oral capsules of Lactobacillus rhamnosus GR-1 and a related strain taken daily for 60 days led to normal lactobacilli colonization in 37% of women, compared to 13% on placebo. Another trial found that women who used a three-strain probiotic mixture after antibiotic treatment had a BV recurrence rate of 18.3%, compared to 32.1% for those on placebo.
The strains with the most evidence behind them include L. crispatus, L. rhamnosus GR-1, L. reuteri RC-14, and L. acidophilus. These are available both orally and as vaginal preparations, though the optimal delivery method and duration are still being refined.
Beyond probiotics, practical steps that support vaginal flora include avoiding douching, using condoms (especially with new partners), and choosing unscented products for intimate hygiene. None of these guarantee prevention, but they reduce the likelihood of the microbial disruption that starts the cascade.
Why Treatment Matters
Left unaddressed, BV does more than cause discomfort. It increases susceptibility to sexually transmitted infections by compromising the vaginal lining’s natural defenses. During pregnancy, the stakes are higher: a meta-analysis found that BV roughly doubles the odds of preterm birth. That association is strong enough that screening and treatment during pregnancy has been recommended as a strategy to reduce preterm delivery rates and neonatal complications.
BV can also increase the risk of infection after gynecological procedures and has been linked to pelvic inflammatory disease. Even in non-pregnant individuals, the chronic inflammation associated with recurring BV is not benign, making effective management worth pursuing even when symptoms feel mild.