Fishy-smelling discharge is most commonly caused by bacterial vaginosis (BV), a condition where the natural balance of bacteria in the vagina shifts. Roughly 23 to 29% of women of reproductive age have BV at any given time, making it the single most common cause of abnormal vaginal discharge. The good news: it’s treatable, and understanding what’s happening helps you recognize it early.
What Causes the Fishy Smell
A healthy vagina is dominated by a type of bacteria called lactobacilli, which produce hydrogen peroxide and keep the environment slightly acidic. When those protective bacteria decline, anaerobic bacteria (the kind that thrive without oxygen) multiply rapidly. These bacteria break down certain compounds in vaginal fluid into a chemical called trimethylamine, which is the same molecule responsible for the smell of spoiling fish.
The process typically starts when one specific bacterium creates a sticky film along the vaginal walls. That film then becomes a hospitable environment for other opportunistic bacteria to move in and multiply. The result is a dramatic shift in the vaginal ecosystem: fewer protective bacteria, more odor-producing ones, and a rise in vaginal pH from its normal acidic range to above 4.5.
Why the Smell Gets Worse at Certain Times
Many people notice the fishy odor is strongest after sex or during their period, and there’s a straightforward chemical reason for both. Semen has a pH of about 8.0, which is significantly more alkaline than the vagina’s normal environment. When semen raises the vaginal pH, it releases the odor-causing amines that were trapped in the acidic fluid. That elevated pH can persist for 10 to 14 hours after unprotected intercourse, which is why the smell often lingers.
Menstrual blood is also slightly alkaline. When it sits against vaginal tissue on a pad or tampon, it raises the local pH in the same way, triggering the same release of fishy-smelling compounds. If you’ve noticed the odor seems to come and go with your cycle or your sexual activity, this pH mechanism is almost certainly why.
What BV Discharge Looks and Feels Like
BV discharge has a distinct profile: thin, homogeneous (uniform in texture rather than clumpy), and grayish-white. It coats the vaginal walls evenly rather than collecting in thick patches. Some people also notice mild pelvic discomfort, but many have no symptoms beyond the smell and the discharge itself. Itching is not a hallmark of BV, which helps distinguish it from yeast infections.
Other Possible Causes
BV isn’t the only infection that can produce a noticeable odor, though it’s by far the most common one. Trichomoniasis, a sexually transmitted infection caused by a parasite, also produces foul-smelling discharge. The key differences are in the discharge itself: trichomoniasis typically causes frothy, yellowish-green discharge rather than the thin gray-white of BV. Trichomoniasis is also more likely to cause itching, irritation, and pain during urination.
A forgotten tampon or other retained foreign body can also produce a strong fishy or rotten odor. This is worth checking before assuming you have an infection, as the fix is simply removal.
Why It Matters Beyond the Smell
BV is not just an annoyance. Left untreated, it increases vulnerability to sexually transmitted infections and can contribute to pelvic inflammatory disease. The risks are particularly significant during pregnancy: BV nearly triples the odds of preterm delivery. One study found that women with BV during pregnancy had a preterm birth rate of 22.7% compared to 6.2% in women without it, and their infants were more likely to need intensive care after birth. If you’re pregnant or planning to become pregnant, treating BV promptly is especially important.
How BV Is Diagnosed
A healthcare provider can usually diagnose BV during a standard pelvic exam using a few straightforward checks: examining the discharge for its characteristic thin, grayish appearance; testing the vaginal pH (above 4.5 suggests BV); looking at a sample under a microscope for “clue cells,” which are vaginal cells coated in bacteria; and performing what’s called a whiff test, where a chemical solution is added to a discharge sample to see if it releases a fishy odor. Meeting three of these four criteria confirms the diagnosis.
Treatment and Recovery
BV is treated with prescription antibiotics, typically taken orally for seven days or applied as a vaginal gel or cream for five to seven days. Most people notice the odor improving within a few days of starting treatment. You don’t need to do anything complicated during treatment, though you may be advised to avoid alcohol if you’re taking the oral form, as the combination can cause nausea.
The frustrating part of BV is its tendency to come back. Recurrence is common, with some estimates suggesting nearly half of treated cases return within months. For people dealing with repeated episodes, combining standard antibiotic treatment with vaginal boric acid suppositories has shown promise. One study tracking patients for 12 months found that this combination approach cut the recurrence rate from 48% to 28% compared to antibiotics alone.
Reducing Your Risk of Recurrence
Because BV is fundamentally about the vaginal ecosystem losing its balance, prevention centers on protecting that ecosystem. Douching is one of the most well-established risk factors, as it strips away the protective lactobacilli and raises vaginal pH. Scented soaps, bubble baths, and vaginal deodorants can do the same thing. The vagina is self-cleaning, and the less you introduce into it, the better its bacterial balance tends to be.
Using condoms helps by preventing semen from repeatedly raising your vaginal pH. Changing out of wet swimwear promptly and wearing breathable cotton underwear reduce the warm, moist conditions that favor anaerobic bacteria. None of these steps guarantee you won’t get BV, but they meaningfully lower the odds, especially if you’ve had it before.