A fishy smell in semen is not normal and usually points to one of a few identifiable causes: a bacterial issue, a dietary factor, or something as simple as hygiene. Healthy semen is slightly alkaline, with a pH between 7.2 and 7.8, and typically has a mild, slightly bleach-like or chlorine smell due to its chemical makeup. A distinctly fishy odor signals that something has shifted.
Bacterial Infections in the Prostate or Urethra
The most common medical explanation is a bacterial infection somewhere along the reproductive tract. The prostate gland contributes a significant portion of seminal fluid, and when it becomes infected (a condition called prostatitis), the chemistry of that fluid changes. During a prostate infection, levels of protective compounds like zinc, magnesium, and natural antibacterial factors drop, while the pH of prostatic fluid can rise as high as 8.0. This more alkaline, less protected environment allows bacteria to thrive and produce foul-smelling byproducts that end up in your ejaculate.
The urethra also harbors its own community of bacteria. Semen passes through the urethra on its way out, picking up whatever microbes live there. A healthy urethral environment contains bacteria like Lactobacillus and Corynebacteria, but when that balance tips toward organisms like Prevotella or Ureaplasma, the result can include changes in smell. Sexually transmitted infections like trichomoniasis can also cause unusual discharge in men, though this infection is more commonly associated with fishy odor in women. In men, trichomoniasis more often causes irritation, burning after urination, or a clear discharge.
What You Eat Can Change How You Smell
Certain foods and supplements produce a compound called trimethylamine in your gut during digestion. Normally, your liver converts trimethylamine into an odorless form before it can accumulate. But when you consume large amounts of the nutrients that feed this process, particularly choline and carnitine, your body may produce more trimethylamine than your liver can handle. The excess gets released through sweat, urine, breath, and reproductive fluids, carrying a distinctly fishy smell.
Foods high in choline include eggs, liver, and certain legumes. Carnitine is concentrated in red meat and is also a popular workout supplement. Fish oil supplements are another common source. If the smell appeared around the same time you changed your diet or started a new supplement, that connection is worth investigating. Cutting back on these foods for a week or two can help you determine whether diet is the cause.
Trimethylaminuria: A Rare Metabolic Condition
Some people have a genetic variation that permanently impairs their ability to break down trimethylamine. This condition, called trimethylaminuria or “fish odor syndrome,” involves a faulty version of the liver enzyme responsible for neutralizing trimethylamine. When this enzyme doesn’t work properly, trimethylamine builds up in the body and is released through sweat, urine, reproductive fluids, and breath, all carrying a strong fishy odor.
There’s also a secondary form of this condition where the enzyme works but something overwhelms it. Taking high doses of choline or carnitine supplements can trigger this in people who otherwise wouldn’t have symptoms. If the fishy smell isn’t limited to your semen and you notice it in your sweat or urine too, trimethylaminuria is worth discussing with a doctor. The condition can be confirmed through urine testing.
Hygiene and Bacterial Buildup
Sometimes the smell isn’t coming from the semen itself but from the skin it passes over during ejaculation. Smegma, a natural buildup of oils, dead skin cells, and sweat that collects under the foreskin or around the glans, creates a warm, moist environment where bacteria multiply. As those bacteria feed and grow, they produce strong odors that can mix with semen and change its perceived smell. Cleveland Clinic describes the odor as similar to sour milk, though it can take on a fishy quality depending on the specific bacteria involved.
Regular washing with mild soap and water, including retracting the foreskin to clean underneath, is usually enough to resolve this. If the smell goes away after improving your hygiene routine, the semen itself was likely fine all along.
Residual Odor From a Partner
If you’re noticing the smell during or after sex rather than from semen on its own, the source may be your partner’s vaginal bacteria rather than your ejaculate. Bacterial vaginosis, a common imbalance of vaginal bacteria, produces a fishy odor that becomes more noticeable after intercourse. The alkaline pH of semen triggers the release of volatile compounds from BV-related discharge, intensifying the smell. In this scenario, your semen acts as a chemical trigger for an odor that originates elsewhere. The distinction matters: if you only notice the fishy smell during partnered sex, BV in your partner is a likely explanation.
How to Narrow Down the Cause
Start with the simplest explanations. Improve your genital hygiene for a few days and see if the smell changes. Review any supplements you’re taking, especially fish oil, choline, or carnitine, and consider pausing them temporarily. Pay attention to whether the smell is present when you ejaculate alone or only during sex with a partner.
If the fishy odor persists after addressing hygiene and diet, or if it comes with other symptoms like burning during urination, unusual discharge, pain in the lower abdomen or groin, or discolored semen, a bacterial infection in the prostate or urethra is the most likely cause. A simple urine test or semen culture can identify the responsible bacteria and guide treatment. If the smell extends beyond your semen to your sweat and breath, testing for trimethylaminuria can rule out or confirm a metabolic cause.