Bacterial vaginosis (BV) comes from a shift in the natural balance of bacteria inside the vagina. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment and keep it slightly acidic, with a pH between 3.8 and 4.5. When those protective bacteria decline and are replaced by an overgrowth of other organisms, BV develops. It’s the most common vaginal condition in women ages 15 to 44, and despite how frequently it occurs, the exact trigger that kicks off this bacterial shift isn’t fully understood.
What Happens Inside the Vagina
A healthy vagina is home to a community of microorganisms, and lactobacilli are the dominant species. These bacteria produce hydrogen peroxide and lactic acid, which maintain the acidic environment that keeps harmful organisms in check. BV occurs when lactobacilli suddenly become scarce and a mix of anaerobic bacteria (organisms that thrive without oxygen) take over. The most commonly identified species in this overgrowth include Gardnerella vaginalis, Atopobium vaginae, Prevotella, Mobiluncus, and Mycoplasma hominis, among others.
As lactobacilli disappear, the vagina’s pH rises and becomes more alkaline. This higher pH creates a more hospitable environment for anaerobic bacteria, which multiply rapidly. The result is the hallmark symptoms of BV: thin grayish-white discharge with a distinct fishy odor, particularly noticeable after sex. Some women, though, have no symptoms at all.
Known Triggers and Risk Factors
While the initial cause of the bacterial shift remains unclear, several factors are strongly linked to disrupting the vaginal microbiome.
Douching is one of the most well-established risk factors. Rinsing the vagina with vinegar, antiseptics, or commercial douching products causes inflammation and can directly eliminate lactobacilli. One study found that douches containing povidone-iodine had a particularly strong inhibitory effect on protective vaginal bacteria compared to saline or acetic acid solutions. The vagina is self-cleaning, and douching disrupts that process.
Scented soaps and antibacterial products used in or around the vaginal area spur bacterial growth by altering the natural chemical environment. Heavily perfumed body washes, bubble baths, and antibacterial soaps are common culprits.
Tight, non-breathable clothing also plays a role. Nylon underwear and tight-fitting workout pants trap moisture and block airflow, creating warm, damp conditions that encourage anaerobic bacteria to flourish. The same applies to staying in sweaty clothes after exercise.
Interestingly, one large study found that deodorized tampons and pads did not increase BV prevalence compared to their unscented versions, despite common assumptions.
The Role of Sexual Activity
BV is not classified as a sexually transmitted infection, but sexual activity is clearly associated with it. New sexual partners, multiple partners, and sex between women all increase the risk. The connection likely involves the introduction of new bacteria or changes to vaginal pH from semen, which is alkaline.
The relationship between BV and sexual transmission is complicated. The CDC notes that whether BV results from acquiring a single sexually transmitted organism is still unknown. Adding to the confusion, data from clinical trials indicate that treating a woman’s male sex partner does not improve her response to therapy or reduce the likelihood of recurrence. This is one of the key reasons BV is not treated the same way as a traditional STI, even though sex can trigger it.
Why BV Keeps Coming Back
One of the most frustrating aspects of BV is its tendency to recur. Between 50 and 80 percent of women experience a recurrence within a year of completing antibiotic treatment. That’s an unusually high failure rate, and it points to something deeper than a simple infection.
Several explanations have been proposed. The anaerobic bacteria involved in BV form biofilms, sticky layers of microorganisms that cling to the vaginal walls and resist antibiotics. Even after treatment kills off most of the overgrowth, a biofilm can serve as a reservoir that reignites the imbalance. Re-exposure to BV-associated organisms through sexual contact is another possible driver. Perhaps the most significant factor is that antibiotics eliminate the bad bacteria but don’t help lactobacilli recolonize the vagina. Without the return of those protective bacteria, the conditions that led to BV in the first place remain.
What BV Can Lead To
BV is not just a nuisance. The disruption in vaginal bacteria increases susceptibility to sexually transmitted infections, including HIV, chlamydia, and gonorrhea. The loss of the acidic, lactobacilli-rich environment removes a key barrier that normally helps prevent these pathogens from establishing an infection.
During pregnancy, BV raises the risk of preterm birth and low birth weight. The inflammatory environment associated with bacterial overgrowth can affect the uterus and amniotic fluid. BV also increases the chance of infection after gynecological procedures like IUD insertion or hysterectomy.
Reducing Your Risk
Because the exact trigger for BV is still unknown, prevention focuses on protecting the vaginal microbiome from disruption. Skip douching entirely. Use mild, unscented soap only on the external genital area, and avoid getting any soap inside the vagina. Choose cotton underwear over synthetic fabrics, and change out of sweaty workout clothes promptly.
Using condoms during sex may help by preventing semen from raising vaginal pH, though the evidence is not definitive. If you notice BV recurring in a pattern, whether after your period, after sex with a specific partner, or after using certain products, that pattern itself can be useful information for identifying your personal triggers.