The most common cause of a fishy vaginal odor is bacterial vaginosis (BV), a condition where the balance of bacteria in the vagina shifts away from protective species toward odor-producing ones. A handful of other causes can produce a similar smell, including certain sexually transmitted infections, a forgotten tampon, and even a rare genetic condition. Understanding what’s behind the odor helps you figure out what to do about it.
Bacterial Vaginosis: The Leading Cause
BV accounts for the vast majority of fishy vaginal odor cases. It happens when the vagina’s normal population of Lactobacillus bacteria, which keep the environment acidic and protective, gets overtaken by other types of bacteria. A healthy vagina has a pH between 3.8 and 4.5. When BV develops, the pH climbs above 4.5, creating conditions where odor-producing bacteria thrive.
The fishy smell itself comes from chemicals called biogenic amines, particularly trimethylamine, the same compound responsible for the smell of rotting fish. Bacteria produce these amines by breaking down amino acids. As they do so, they also raise the pH of the vaginal environment even further, which lets more of those bacteria grow. It’s a self-reinforcing cycle: the bacteria shift the pH, and the higher pH favors more bacteria. This is why the odor often gets noticeably stronger after sex or during a period, since semen and blood are both alkaline and temporarily push the pH higher, releasing more of those volatile amines.
Beyond the smell, BV often causes a thin, grayish-white discharge. Some people notice mild itching, but many have no symptoms other than the odor. BV is not a sexually transmitted infection, though sexual activity can increase the risk of developing it. Douching, using scented soaps inside the vagina, and having new or multiple sexual partners are all associated with disrupting the bacterial balance.
Trichomoniasis and Other Infections
Trichomoniasis, a sexually transmitted infection caused by a tiny parasite called Trichomonas vaginalis, can also produce a foul or fishy smell. The discharge tends to look different from BV: it’s often yellow-green, frothy, and accompanied by more noticeable irritation, itching, or burning during urination. Not everyone with trichomoniasis has symptoms, though. Up to 70% of infected people don’t notice anything at first, which means the infection can go undetected for weeks or longer.
Because BV and trichomoniasis can look and smell similar, testing is the only reliable way to tell them apart. Treatment differs for each, so getting the right diagnosis matters.
A Forgotten Tampon or Foreign Object
A retained tampon is a surprisingly common and often overlooked cause of strong vaginal odor. When a tampon is left in place beyond the recommended six to eight hours, bacteria begin breaking down the trapped blood and tissue, producing an intensely foul smell that’s hard to ignore. The discharge is typically yellow, pink, or brown and gets worse the longer the object stays in place.
This can happen more easily than you’d think. A tampon can get pushed to the back of the vaginal canal during sex or simply be forgotten at the end of a period. Once removed, the odor usually resolves within a day or two without any other treatment, though an infection can develop if it’s been left in for an extended period.
How Diet and Lifestyle Affect Vaginal Odor
What you eat can influence the bacterial balance in your vagina. Research has found that higher intake of animal protein and alcohol is linked to vaginal dysbiosis, the same type of bacterial imbalance seen in BV. Alcohol consumption specifically correlates with higher levels of Gardnerella, one of the key bacteria involved in producing that fishy odor. Animal protein may worsen the vaginal environment by increasing inflammation and producing ammonia or sulfides during digestion.
On the other hand, diets higher in fiber, vegetable protein, and whole carbohydrates are associated with lower levels of Gardnerella. Plant-based omega-3 fatty acids, found in foods like walnuts, flaxseeds, and chia seeds, appear to support a Lactobacillus-dominant vaginal environment. These beneficial bacteria produce lactic acid that keeps the pH low and crowds out odor-causing species.
Trimethylaminuria: A Rare Genetic Cause
In rare cases, a persistent fishy body odor, including from the vaginal area, has nothing to do with infection at all. Trimethylaminuria is a genetic condition where the body can’t break down trimethylamine, the same chemical that bacteria produce in BV. Normally, a liver enzyme converts trimethylamine into an odorless compound. People with variants in the gene for this enzyme accumulate trimethylamine in their sweat, urine, breath, and vaginal secretions.
The odor in trimethylaminuria is constant or recurrent regardless of vaginal bacterial balance, and it doesn’t respond to antibiotics. It tends to worsen after eating foods rich in the building blocks of trimethylamine, such as eggs, fish, liver, and legumes. If you’ve been treated for BV multiple times without the smell resolving, this is worth discussing with a healthcare provider. A urine test can confirm whether trimethylamine levels are elevated.
How BV Is Treated
BV is treated with prescription antibiotics, typically taken either as an oral pill or an intravaginal gel or cream. A standard course runs five to seven days. Most people notice the odor fading within the first few days of treatment. Trichomoniasis requires a different antibiotic, which is why accurate diagnosis is important before starting any medication.
One frustrating aspect of BV is its tendency to come back. Recurrence rates are high, with many people experiencing repeat episodes within months of treatment. This is where long-term prevention becomes important.
Preventing Recurrence
Keeping your vaginal environment favorable for Lactobacillus bacteria is the most effective way to prevent fishy odor from returning. Several practical strategies help with this:
- Avoid douching and internal fragrances. These strip away protective bacteria and raise vaginal pH, setting the stage for BV.
- Consider probiotics. Clinical trials have shown that specific Lactobacillus strains, including L. rhamnosus GR-1 and L. acidophilus, can reduce BV recurrence and help restore normal vaginal bacteria. These have been studied both as oral supplements taken for two months and as intravaginal treatments used for six to twelve days.
- Adjust your diet. Increasing fiber, whole grains, and plant-based omega-3s while reducing alcohol intake supports a healthier vaginal microbiome.
- Use condoms with new partners. Exposure to new bacteria through sexual contact is one of the most consistent risk factors for BV.
A mild, slightly acidic or musky vaginal scent is completely normal and varies throughout your menstrual cycle. The odor worth paying attention to is specifically fishy, persistent, or accompanied by unusual discharge. That pattern points to a bacterial shift that’s unlikely to resolve on its own and typically needs treatment to correct.