A fishy smell from the vaginal area is most commonly caused by bacterial vaginosis (BV), a condition where the normal balance of bacteria in the vagina shifts and allows odor-producing bacteria to take over. BV affects roughly 23 to 29% of women of reproductive age worldwide, making it the single most common cause of unusual vaginal discharge and odor. While BV is the leading explanation, a handful of other causes can produce a similar smell.
Why the Smell Is Fishy, Specifically
The fishy odor isn’t random. It comes from specific chemicals called amines that certain bacteria produce as they grow. The main culprit is trimethylamine, the same compound responsible for the smell of rotting fish. Bacteria associated with BV, particularly a species called Gardnerella vaginalis, generate trimethylamine along with other amines like putrescine and cadaverine. Women with a healthy vaginal bacterial balance have only trace amounts of these chemicals, while women with BV have significantly elevated levels.
These amines become more noticeable in alkaline (less acidic) conditions. That’s why many women notice the smell is strongest after sex. Semen is alkaline, with a pH around 7 to 8, and when it mixes with vaginal fluid it temporarily raises the pH and releases more of those volatile amines into the air. The same intensifying effect can happen during your period, since blood is also slightly alkaline.
Bacterial Vaginosis: The Most Likely Cause
A healthy vagina is kept acidic (pH 3.5 to 4.5) by beneficial bacteria, primarily Lactobacillus species, which produce lactic acid and hydrogen peroxide. When something disrupts that population, opportunistic bacteria move in, the pH climbs above 4.5, and symptoms develop. BV isn’t a single infection caused by one invading germ. It’s an imbalance, a shift in the entire microbial community.
The hallmark symptoms are a thin, white or gray discharge and a strong fishy odor. The discharge is typically not thick or clumpy. Many women with BV have no other symptoms at all, no itching, no burning, just the smell. Things that can trigger the imbalance include douching, new sexual partners, smoking, and the use of scented soaps or products inside the vagina.
BV is treatable and curable. Standard treatment involves a course of antibiotics, either taken orally or applied as a vaginal gel or cream, typically for five to seven days. The odor usually resolves within a few days of starting treatment. The challenge with BV is recurrence. Up to half of women who are treated experience a return of symptoms within a year.
Reducing Recurrence
Researchers have been investigating whether probiotics can help prevent BV from coming back. The most promising results involve a Lactobacillus strain called L. crispatus, which is considered the most protective species for vaginal health. In clinical trials, an intravaginal L. crispatus suppository cut the recurrence rate of BV roughly in half compared to placebo. Oral probiotics containing strains like L. rhamnosus and L. reuteri have shown mixed results. Some small studies found they helped restore vaginal flora, but a larger trial in pregnant women with BV found no significant benefit over placebo. The science is still catching up to the marketing on this one.
Trichomoniasis
Trichomoniasis is a sexually transmitted infection caused by a parasite, and it can produce a fishy smell that overlaps with BV. The discharge may be clear or white with a fishy odor, though it can also appear yellowish or greenish and may be frothy. Unlike BV, trichomoniasis often comes with additional symptoms: itching, irritation, burning during urination, and discomfort during sex. The only way to tell the two apart with certainty is through testing, since the odor alone isn’t a reliable distinguisher.
A Retained Tampon or Other Object
A forgotten tampon is a surprisingly common cause of a sudden, intensely foul vaginal odor. The smell tends to come on within a few days and is often described as overwhelmingly bad, sometimes worse than a typical BV odor. Along with the smell, you may notice unusual discharge that can be yellow, green, pink, gray, or brown. Other warning signs include fever, pelvic pain, swelling, redness, and pain when urinating.
If left in place for an extended period, a retained object can lead to infection and, in rare cases, more serious complications. If you suspect a forgotten tampon or another object, try to remove it gently yourself. If you can’t reach it or you’re experiencing fever or significant pain, have a healthcare provider remove it.
Sweat and Normal Body Odor
Not every odor from the vaginal area originates inside the vagina. The vulva and groin are rich in apocrine sweat glands, the same type found in your armpits. These glands produce sweat that is thicker and higher in fat and protein than sweat from the rest of your body. When that sweat interacts with the bacteria living on the skin’s surface, it produces a stronger smell. Tight clothing, synthetic fabrics, exercise, and hot weather can all trap this sweat and intensify the odor.
This type of smell is not fishy in the classic BV sense, but it can be musky or pungent enough to be noticeable through clothing. The smell of trapped sweat can also mix with the natural scent of the vaginal microbiome, creating an unfamiliar odor that might cause concern. Wearing breathable cotton underwear, changing out of sweaty workout clothes promptly, and washing the external vulvar area with plain warm water are usually enough to manage it.
Hormonal and Menstrual Cycle Changes
Your vaginal pH naturally fluctuates throughout your cycle. It tends to rise just before your period and stays elevated during menstruation, which can temporarily shift the bacterial balance and produce a more noticeable odor. After menopause, estrogen levels drop, the vaginal walls thin, and pH rises above 4.5 on a more permanent basis. This higher pH can make postmenopausal women more susceptible to BV and related odor changes.
How the Cause Is Identified
A healthcare provider will typically take a sample of vaginal discharge and check your vaginal pH. They’re looking for elevated levels of Gardnerella vaginalis and the presence of “clue cells” under a microscope, which are the hallmark of BV. They may also test for trichomoniasis and yeast infections to rule those out, since the symptoms can overlap. The visit is straightforward and results are often available quickly.
It’s particularly important to get evaluated if you’re pregnant, since BV during pregnancy is associated with a higher risk of preterm birth, low birth weight, and infection of the amniotic fluid. If the odor is accompanied by fever, pelvic pain, or significant changes in discharge color, those warrant prompt attention regardless of pregnancy status.