Why Does My Asshole Smell Like Fish?

A persistent fishy smell from the anal area usually comes from bacteria breaking down sweat, skin oils, or traces of stool in one of the body’s most microbe-dense regions. The perianal skin sits in a warm, moist fold packed with specialized sweat glands and a rich bacterial population, making it one of the places most prone to strong odors. But while everyday hygiene issues account for most cases, a fishy smell specifically can also signal infections, skin conditions, metabolic disorders, or structural problems worth investigating.

Why That Area Smells Stronger Than the Rest of Your Body

Your perianal skin is home to apocrine sweat glands, the same type found in armpits and the groin. These glands produce a thick, oily secretion that is essentially odorless on its own but becomes pungent when bacteria on the skin surface break it down into volatile compounds.¹ Add to that the proximity to fecal matter, the lack of airflow between the buttocks, and the friction of sitting all day, and you have an environment custom-built for producing smell.

Resident bacteria on the skin surface are the primary drivers of body odor in general. They metabolize the fats and proteins in sweat and skin secretions, producing small volatile molecules that your nose registers as anything from sour to fishy.² Which specific smell you get depends on which bacterial species dominate and what substrates they have to work with. A fishy note, specifically, is often tied to the production of trimethylamine (TMA) or related amines, which is where things get more interesting.

Trimethylamine and the Gut Connection

Trimethylamine is the molecule responsible for what we recognize as “fishy.” It is produced in the gut when bacteria ferment nutrients like choline, carnitine, and betaine, compounds found in eggs, red meat, fish, and certain legumes. Researchers have identified specific gut bacteria that convert dietary carnitine into TMA through a dedicated set of enzymes, even in the oxygen-free environment of the colon.³ Normally, the liver converts TMA into an odorless compound called TMAO before it can cause trouble. But if TMA production spikes, if you eat a lot of TMA-precursor foods, or if liver processing falters, the excess TMA gets excreted through sweat, urine, and breath, often giving the perianal and genital areas a pronounced fishy smell.

For most people, a strong-smelling day after a seafood-heavy meal or a protein binge is just the gut microbiome doing its normal work at a higher volume. The smell passes as the food clears your system. But for a small number of people, the fishy smell is chronic and unrelenting, pointing to a genetic condition.

Trimethylaminuria, the Fish Odor Syndrome

Trimethylaminuria (TMAU) is a metabolic disorder in which the body cannot adequately convert TMA to TMAO. The enzyme responsible, called FMO3, is either missing or significantly impaired due to mutations in the gene that codes for it. In its most severe form, a mutation can completely abolish the enzyme’s activity.⁴ Other variants leave the enzyme partially functional, meaning the odor may come and go depending on diet, hormonal state, and overall health.⁵

TMAU is diagnosed by measuring TMA and TMAO concentrations in urine, typically using specialized lab techniques.⁶ Sometimes doctors administer a choline loading test, where you take a large dose of choline and then provide urine samples to see how well your body handles the resulting TMA.⁷ The distinction between primary TMAU (caused by genetic mutations) and secondary TMAU (caused by liver disease, gut dysbiosis, or certain medications) matters for treatment, and the two forms produce different urinary patterns of TMA and TMAO.⁸

If TMAU is confirmed, dietary restrictions that limit choline, carnitine, and lecithin can help reduce the smell. In at least some cases, supplementing with riboflavin (vitamin B2) has proven remarkably effective. One documented case showed that taking 200 mg of riboflavin daily produced an immediate improvement in odor noticed by the patient’s friends and family, with continued reductions over time.⁹ Riboflavin is a cofactor that helps the FMO3 enzyme work better, so when the enzyme is impaired rather than completely absent, boosting riboflavin can partially compensate.

Infections That Produce Fishy or Foul Discharge

A fishy smell localized to the anus can also come from an infection in the rectal lining. Infectious proctitis, an inflammation of the rectum caused by sexually transmitted organisms, is a common culprit in people who have receptive anal intercourse. The most frequently identified agents are gonorrhea, chlamydia, herpes simplex virus, and syphilis.¹⁰ These infections can produce mucous discharge, pain, and a strong odor. What makes them tricky is that the symptoms can mimic inflammatory bowel disease, which means some people end up being treated for the wrong condition.¹¹

If the fishy smell appeared relatively recently, especially alongside rectal discharge, pain, or a change in bowel habits, an STI screen is worth pursuing. Gonorrheal proctitis in particular can produce a mucous, foul-smelling discharge that people sometimes mistake for a hygiene issue or hemorrhoid complication. Treatment is straightforward with the right antibiotics, but it will not resolve on its own.

Abscesses, Fistulas, and Pilonidal Cysts

Structural problems in and around the anus can also be the source. Perianal abscesses typically start as infections in the tiny glands inside the anal canal, then spread into adjacent tissue. About four in ten abscesses go on to form fistulas, which are abnormal tunnels between the inside of the anal canal and the skin surface.¹² Fistulas can drain pus, mucus, or even small amounts of stool through a tiny opening near the anus, producing a persistent foul smell that no amount of washing fixes. The drainage may be intermittent, sometimes going away for days before returning.

Pilonidal disease is another possibility, especially if the smell seems to come from higher up in the crease between the buttocks rather than from the anus itself. A pilonidal sinus forms when hair follicles in the natal cleft become infected, creating a cavity that fills with debris and drains malodorous fluid.¹³ It can present as a small dimple or opening in the skin above the tailbone, sometimes with swelling, pain, and intermittent discharge. Both fistulas and pilonidal cysts generally require medical or surgical treatment; they rarely resolve with hygiene changes alone.

Fungal Overgrowth in Skin Folds

Candidal intertrigo, a yeast infection in skin folds, is worth considering if the smell comes with redness, maceration (wet, whitish skin), or itching in the crease of the buttocks. Candida thrives in warm, moist skin folds and can produce a distinctive musty or sour odor that some people describe as fishy. Obesity, diabetes, and anything that keeps the area damp (tight clothing, heavy sweating, limited mobility) all raise the risk.¹⁴ Recurrent cases often need more than just antifungal cream; addressing the underlying conditions and keeping the skin folds dry are critical for breaking the cycle.

When Vaginal Issues Spread to the Perianal Area

For women, the proximity of the vagina to the anus means that vaginal infections can sometimes be perceived as anal odor, or can genuinely affect the perianal skin. Bacterial vaginosis, the most common cause of a fishy vaginal smell, is caused by an overgrowth of certain anaerobic bacteria. Some of those organisms also show up in the perianal microbiome. However, research suggests that BV is not simply a case of intestinal bacteria migrating to the vagina; only certain species display a strong tendency to colonize the vaginal environment, while many other gut species do not.¹⁵

If you are a woman experiencing a fishy smell and cannot pinpoint whether it is coming from the vaginal or anal area, a doctor can help differentiate with a simple exam and swab tests. The treatments are different, so getting the location right matters.

Fecal Incontinence and Mucous Leakage

Sometimes the explanation is mechanical rather than infectious or metabolic. Even minor fecal incontinence, where small amounts of stool or mucus leak without your full awareness, can leave a persistent smell. This is more common than most people realize, and it is not limited to elderly or disabled individuals. Risk factors include prior childbirth injuries, surgical history, diabetes, inflammatory bowel disease, and reduced physical activity.¹⁶ Some people have reduced rectal sensation and do not feel the leakage happening. If you notice staining on underwear or a smell that seems to return quickly after cleaning, bringing this up with a doctor is a good step. Treatments range from dietary fiber adjustments and pelvic floor exercises to medical management of the underlying cause.

Hygiene Practices and Over-Washing

It is natural to assume the fix is more washing, but the relationship between soap use and skin microbiome is more nuanced than scrub-harder logic suggests. Research on antibacterial soap use shows that while it does not necessarily reduce the overall number of bacterial species on the skin, it can shift which species dominate, and those shifts can persist for weeks after you stop using the soap.¹⁷ In theory, aggressive scrubbing with harsh soaps could reduce protective bacteria and allow odor-producing species to flourish, though this has not been specifically studied for the perianal area.

The practical takeaway: gentle cleaning with water and a mild, unscented soap is generally better than aggressive scrubbing with antibacterial products. Drying the area thoroughly matters too, since lingering moisture fuels bacterial and fungal growth. A bidet or a handheld spray followed by gentle patting dry is a reasonable approach. Some people find that a thin application of zinc oxide barrier cream helps keep the skin dry and reduces odor throughout the day, particularly if sweating is a major contributor.

When the Smell Might Not Be Real

This is a sensitive topic, but it needs to be addressed honestly. A condition called olfactory reference syndrome (ORS) involves a persistent preoccupation with a perceived body odor that other people cannot detect. People with ORS most commonly report believing they smell like stool, garbage, or ammonia, and the odor is often perceived as coming from the armpits, feet, or genital and anal area.¹⁸

ORS is not a quirky worry. It can be severely disabling. Research has found that roughly three-quarters of affected individuals avoid most social interactions during symptomatic periods, and about 40% have been completely housebound for at least a week because of their symptoms. Rates of suicidal ideation are high, and many attribute those thoughts primarily to the odor preoccupation.¹⁹ The average age of onset is around 21, and more than half of people with ORS describe it as chronic and unremitting. Nearly all engage in time-consuming rituals to check for or camouflage the perceived smell.¹⁸

If you have asked trusted people whether they notice the smell and they consistently say no, if no medical workup has turned up a cause, and if the worry about the smell is affecting your social life or mental health, it is worth raising the possibility of ORS with a mental health professional. This is treatable, typically with a combination of cognitive behavioral therapy and, in some cases, medication. Raising it does not mean dismissing your experience; it means taking it seriously enough to consider every explanation.

How Doctors Figure Out the Cause

If the smell is persistent and you cannot identify an obvious trigger, a doctor will typically start with a physical exam of the perianal area, looking for fissures, fistula openings, signs of infection, or skin changes. Swab tests can check for STIs and fungal organisms. If metabolic causes are suspected, a urine test for TMA and TMAO ratios is the standard diagnostic tool for trimethylaminuria.⁶ Genetic testing can then confirm whether a mutation in the FMO3 gene is present.⁴

The embarrassment factor with this type of complaint is real, and it keeps people from seeking help for far too long. Gastroenterologists, dermatologists, and colorectal specialists have all seen this before. Coming in with a clear description of when the smell started, whether it comes and goes or is constant, whether there is any discharge, and whether anyone else has noticed it will help the conversation move quickly toward answers.

Why the Anal Region Produces Scent Signals at All

There is an evolutionary backdrop to all of this that is oddly reassuring. In most mammals, the anal and perianal region is one of the body’s primary scent-signaling zones. Anal gland secretions in species like badgers encode information about individual identity, sex, reproductive status, and social group membership, all broadcast through volatile compounds deposited alongside feces.²⁰ Similar chemical signaling through anal gland odors varies with age, sex, and social status in other mammals as well.²¹ Humans do not have true anal glands the way these animals do, but we retain the apocrine gland concentration in the perianal area. The fact that this region produces strong odors is not a design flaw; it is an inheritance from ancestors for whom scent was a primary communication channel. Our modern noses and social norms just make us far less tolerant of what evolution thought was useful information.