Bacterial vaginosis (BV) develops when the balance of bacteria in the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate and keep the environment slightly acidic. When something disrupts that balance, other bacteria that were always present in small numbers multiply rapidly and take over. BV affects roughly 23% to 29% of women of reproductive age worldwide, making it the most common vaginal condition.
What Actually Happens Inside the Vagina
A healthy vagina is home to a community of bacteria, with lactobacilli making up the majority. These bacteria produce lactic acid, which keeps vaginal pH low (acidic) and prevents other organisms from growing out of control. BV occurs when lactobacilli are suddenly replaced by a mix of anaerobic bacteria, organisms that thrive in low-oxygen, higher-pH environments.
The exact trigger for this takeover is still unknown. What researchers do know is that once lactobacilli decline, vaginal pH rises, and that more alkaline environment fuels further growth of the replacing bacteria. Some of these bacteria form a sticky layer called a biofilm on the vaginal walls, which is one reason BV is so hard to get rid of permanently. Within 6 to 12 months of finishing antibiotic treatment, 50% to 80% of women experience a recurrence.
Sexual Activity Is the Strongest Risk Factor
BV is not classified as a sexually transmitted infection, but sexual activity is closely linked to getting it. A large meta-analysis found that having a new male partner or multiple male partners increases BV risk by 1.6 times. Having a female sexual partner doubles the risk, and studies of monogamous female couples show BV concordance rates between 73% and 95%, meaning both partners almost always share the same vaginal bacterial profile.
Semen plays a direct role. It has a pH around 7.2 to 8.0, which is significantly more alkaline than the vagina’s typical range of 3.8 to 4.5. After unprotected sex, average vaginal pH rises to about 5.5 within a few hours, and it can remain elevated at 5.2 even 10 to 14 hours later. That temporary pH spike gives BV-associated bacteria a window to multiply. Compounds in semen also directly inhibit lactobacilli growth while promoting the growth of pathogenic bacteria.
Consistent condom use reduces BV risk by about 20%, which aligns with the semen connection. Reducing the number of sexual partners and using condoms consistently are the two most evidence-supported ways to lower your chances of developing BV.
Douching and Scented Products
Douching, the practice of flushing the vagina with water or cleansing solutions, is one of the most well-documented BV risk factors. It physically disrupts the bacterial community inside the vagina, causes inflammation, and creates an opening for harmful bacteria to colonize. Douching has been linked not only to BV but also to pelvic inflammatory disease and preterm birth.
Scented feminine hygiene products can cause similar problems. Lab research found that certain popular vaginal moisturizers and washes suppressed lactobacillus growth within two hours and killed the bacteria entirely within 24 hours. This creates a harmful cycle: women use scented products to manage odor or discharge, the products disrupt the microbiome, symptoms get worse, and the woman washes more frequently. The vaginal odor that many women try to eliminate with these products is, in most cases, completely normal.
Other Factors That Raise Your Risk
Smoking is consistently associated with higher BV rates. Research has found a statistically significant link between tobacco use and BV, though the biological mechanism is not entirely clear. Some evidence suggests that chemicals from tobacco concentrate in cervical mucus and may impair the immune defenses that help maintain a healthy bacterial balance.
A few other factors are worth knowing about:
- Partner circumcision status: Male circumcision reduces BV risk in female partners, likely because the foreskin can harbor BV-associated bacteria.
- Hormonal contraception: Birth control pills and other hormonal methods do not increase BV risk and may actually be protective, possibly because steady hormone levels help maintain a stable vaginal environment.
- IUDs and antibiotics: Broad-spectrum antibiotics can wipe out lactobacilli along with the infection they’re treating, leaving the vagina vulnerable to a bacterial shift.
Why Probiotics Haven’t Solved the Problem
Given that BV is fundamentally a loss of beneficial lactobacilli, taking probiotics to replace them sounds logical. In practice, the CDC notes that no studies support probiotic products as either a replacement or add-on therapy for BV. The challenge is that the lactobacillus strains in most commercial probiotics are not the same strains that naturally dominate the vagina, and even when the right strains are used, they struggle to establish themselves against an existing biofilm of BV bacteria.
What You Can Actually Do
The most effective prevention strategies are straightforward. Use condoms, especially with new partners. Avoid douching entirely. Skip scented washes, sprays, and wipes marketed for vaginal use. Plain water on the external area is sufficient for hygiene. If you smoke, that’s one more reason to quit.
If you notice a thin, grayish-white discharge with a fishy smell, especially after sex, those are the hallmark signs of BV. It’s worth getting tested rather than trying to treat it on your own, because the symptoms overlap with yeast infections and other conditions that require different treatment. BV that goes untreated can increase susceptibility to sexually transmitted infections and, during pregnancy, raise the risk of preterm delivery.