Bacterial vaginosis (BV) happens when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria keep the vaginal environment slightly acidic (a pH between 3.8 and 5), which suppresses harmful microbes. When those protective bacteria decline sharply, anaerobic bacteria multiply and take over. The result is BV, and it’s extremely common.
Despite decades of research, the exact trigger for this bacterial shift isn’t fully understood. But several well-established factors make it far more likely to happen.
What Changes Inside the Vagina
A healthy vagina is dominated by Lactobacillus bacteria, which produce antimicrobial compounds that keep other organisms in check. During BV, Lactobacillus numbers drop dramatically, and anaerobic bacteria surge to fill the gap. The most commonly identified organism in BV is Gardnerella vaginalis, though it usually appears alongside a mix of other anaerobes rather than acting alone.
This shift raises the vaginal pH, making the environment less acidic and more hospitable to harmful bacteria. Once that protective acidity is lost, it can be difficult for Lactobacillus to re-establish dominance on its own, which is one reason BV tends to come back after treatment. Between 50% and 80% of women experience a recurrence within a year of finishing antibiotics.
Sexual Activity Is the Strongest Link
BV is not classified as a sexually transmitted infection, but sexual activity is its most consistent risk factor. Women who have never been sexually active are rarely affected. Having a new sex partner, multiple partners, or unprotected sex all increase the likelihood of developing BV. Among women who have sex with women, there’s a high level of BV concordance between partners, meaning both partners often have it at the same time.
The connection likely comes down to what sex introduces into the vaginal environment. Semen is alkaline and can temporarily raise vaginal pH. Saliva, lubricants, and bacteria from a partner’s skin or genitals can all shift the microbial balance. BV-associated bacteria have been found on male genitalia, though treating male partners hasn’t been shown to prevent recurrence in female partners. This disconnect is part of why researchers still can’t pin BV to a single transmitted pathogen the way they can with chlamydia or gonorrhea.
Douching and Hygiene Products
Douching is one of the clearest preventable risk factors. Women who douche at least once a month have a 1.4 times higher risk of developing BV. That risk jumps to 2.1 times for women who douched within the past week. Douching washes away Lactobacillus and disrupts the vaginal pH, essentially clearing the path for anaerobic bacteria to move in.
Other products can cause similar disruption. Vaginal cleaning products, scented soaps, and certain lubricants can all kill off protective bacteria. The vagina is self-cleaning, and introducing products into it tends to do more harm than good. Even antibiotics taken for unrelated infections can wipe out enough Lactobacillus to trigger BV.
Other Factors That Raise Your Risk
Beyond sex and hygiene practices, several other things can tip the bacterial balance:
- Menstrual cycle changes. Hormonal fluctuations throughout your cycle affect vaginal acidity. Some women notice BV symptoms around their period, when pH naturally rises.
- Intravaginal practices. Inserting herbs, medications, or other products can alter the microbial environment.
- Diet and overall health. Both can influence the composition of vaginal flora, though the relationship is less direct than the factors above.
- HSV-2 infection. Having genital herpes is associated with higher BV rates, though the reasons aren’t entirely clear.
Why BV Keeps Coming Back
One of the most frustrating things about BV is its recurrence rate. Standard antibiotic treatment cures it in roughly 50% to 60% of cases within 30 days, but within 6 to 12 months, the majority of women see it return. The underlying bacterial imbalance often isn’t fully corrected by a single course of treatment.
Probiotics have gotten a lot of attention as a potential solution. The idea is to replenish Lactobacillus directly. However, clinical evidence has been disappointing so far. In one randomized trial, adding oral probiotics to standard antibiotic treatment produced no improvement in cure rates compared to antibiotics alone. Researchers found that the probiotic species were rarely detected in the vaginal microbiota after being taken orally, which likely explains the lack of benefit.
Recurrence is more likely if the original triggers remain. If you’re still using products that disrupt vaginal pH, or if re-exposure to a partner’s bacteria keeps shifting the balance, the same cycle tends to repeat. Reducing those exposures, particularly douching, is the most evidence-backed way to lower recurrence risk.
How BV Is Identified
BV often produces a thin, grayish-white discharge with a noticeable fishy odor, especially after sex. Some women have no symptoms at all and only discover it during a routine exam. A healthcare provider can diagnose it by checking vaginal pH, examining discharge under a microscope, and looking for characteristic changes in the types of cells present.
It’s worth noting that BV symptoms can overlap with yeast infections and other conditions. The discharge patterns differ (BV tends to be thin and grayish, while yeast infections produce thick, white, clumpy discharge), but self-diagnosis is unreliable. Getting tested matters because untreated BV can increase susceptibility to STIs and cause complications during pregnancy.