A fishy smell from your genitals is most commonly caused by bacterial vaginosis (BV), a condition where the normal balance of vaginal bacteria shifts and allows odor-producing organisms to take over. For people with a penis, the most likely culprit is a buildup of smegma or a bacterial skin overgrowth. While the smell can feel alarming, the causes are usually treatable and rarely dangerous.
Bacterial Vaginosis: The Most Common Cause
BV is responsible for the vast majority of fishy vaginal odor. It happens when the healthy bacteria that normally keep the vagina slightly acidic (below a pH of 4.5) get outnumbered by other types of bacteria. These bacteria produce chemical byproducts called putrescine and cadaverine, which are the compounds that create the characteristic fishy smell. No single type of bacteria is fully responsible on its own. It takes a mixed community of organisms working together to generate the odor.
The smell is often strongest after sex or during your period, because semen and blood are both alkaline, which triggers a greater release of those odor-causing compounds. You may also notice a thin, grayish-white discharge that looks different from your normal discharge. Some people with BV have no symptoms at all beyond the smell.
BV is not a sexually transmitted infection, though sexual activity can increase your risk. Other risk factors include douching, which more than doubles BV risk when done within a week beforehand, and using scented soaps or products inside the vagina. Douching disrupts the protective bacteria and creates conditions for the odor-causing organisms to flourish.
Sexually Transmitted Infections
Trichomoniasis, a common STI caused by a parasite, also produces a fishy smell. The CDC describes the discharge as clear, white, yellowish, or greenish, often thin or increased in volume. It may come with itching, burning during urination, or irritation that BV alone doesn’t typically cause. Trichomoniasis is curable with prescription medication, but it won’t resolve on its own.
Other STIs like chlamydia and gonorrhea can cause unusual discharge and odor, though the smell is less distinctly “fishy” than with BV or trichomoniasis. If the odor appeared after a new sexual partner or unprotected sex, STI testing is worth pursuing even if the smell is your only symptom.
Causes in People With a Penis
For uncircumcised people, the most common source of a fishy or sour smell is smegma, a natural mixture of oils, dead skin cells, and sweat that collects under the foreskin. Smegma itself isn’t harmful or infectious. But when it accumulates, it creates an ideal environment for bacteria to grow and feed, and those bacteria produce a strong odor often described as sour or fishy.
The fix is straightforward: gently pull back the foreskin and wash underneath with warm water daily. Harsh soaps can irritate the skin and sometimes make the problem worse. If the smell persists despite good hygiene, or if you notice redness, swelling, or discharge from the tip of the penis, a bacterial or yeast infection may be involved.
A Rarer Possibility: Fish Odor Syndrome
If the fishy smell comes from your sweat, urine, and breath (not just your genitals), you may have trimethylaminuria, a genetic condition sometimes called fish odor syndrome. People with this condition can’t fully break down a compound called trimethylamine, which builds up in the body and gets released through all bodily fluids. The odor is persistent regardless of hygiene and has been present since childhood in most cases. It’s rare, but if a fishy smell affects your whole body rather than just one area, it’s worth mentioning to your doctor.
How a Fishy Odor Gets Diagnosed
Diagnosing BV is quick and painless. A clinician collects a small sample of vaginal discharge and checks several things: whether the vaginal pH has risen above 4.5, whether certain indicator cells are visible under a microscope, and whether the discharge produces a fishy smell when mixed with a chemical solution (called a whiff test). Meeting three of these four criteria confirms BV.
If trichomoniasis or another STI is suspected, a separate swab or urine test can identify the specific organism. These results typically come back within a few days.
Treatment and What to Expect
BV is treated with prescription antibiotics, either taken as pills or applied as a vaginal gel or cream. A typical course lasts five to seven days. Symptoms usually improve within a couple of days of starting treatment, though it’s important to finish the full course. BV has a frustrating tendency to come back. Roughly half of people treated for BV experience a recurrence within 12 months.
Trichomoniasis is also treated with oral antibiotics, and sexual partners need treatment at the same time to prevent reinfection.
For recurring BV, avoiding douching is one of the most effective preventive steps. Women who douche at least once a month have a 1.4 times higher risk of developing BV compared to those who don’t. Switching to unscented soap for external washing and avoiding any products inside the vagina can also help maintain a healthy bacterial balance.
Why It Matters During Pregnancy
BV during pregnancy carries specific risks. It increases the chance of preterm birth (delivery before 37 weeks) and low birthweight (under 5 pounds, 8 ounces). Both outcomes can cause health complications for the baby. If you’re pregnant and notice a fishy odor, getting tested and treated promptly reduces these risks.
Signs the Problem May Be Progressing
A fishy smell on its own is not an emergency, but certain additional symptoms suggest the infection may have spread beyond the vagina into the uterus or fallopian tubes, a condition called pelvic inflammatory disease. Watch for strong pelvic or lower abdominal pain, nausea or vomiting, fever above 101°F, painful urination, or bleeding between periods. These symptoms need prompt medical attention, as untreated PID can lead to lasting damage to the reproductive organs.