Can vitamins cause a fishy smell?

December 9, 2025 •

4 min read

High doses of certain vitamin supplements, particularly those containing choline, have been linked to a rare metabolic condition that can result in a fishy body odor. This phenomenon, often connected to the genetic disorder trimethylaminuria (TMAU), means that while it is not a common side effect, it can affect some individuals who cannot properly metabolize specific compounds.

Quick Summary

An unusual fishy odor may arise from high-dose supplements like choline and carnitine, potentially linked to trimethylaminuria (TMAU), a metabolic issue. Other factors, including rancid fish oil or certain medical conditions, can also be the cause.

Key Points

High-Dose Choline is the Main Culprit: Large supplemental doses of choline and L-carnitine are the primary vitamin-related cause of a fishy body odor.
Trimethylaminuria (TMAU) is the Underlying Condition: The odor often indicates TMAU, a genetic metabolic disorder where the body cannot process the compound trimethylamine (TMA).
Rancid Fish Oil Causes Fishy Burps: A fishy smell from a fish oil supplement, particularly burps, is a sign that the oil has gone bad, not a metabolic issue.
Other B Vitamins Can Alter Odor: Vitamins like B1 contain sulfur and can cause a strong urine odor, distinct from the TMA-related fishy smell.
Professional Diagnosis is Key: If you experience a persistent fishy odor, it is crucial to consult a doctor to rule out or confirm TMAU and other medical conditions.
Dietary Adjustments Can Help: Reducing foods high in choline and carnitine, like eggs and liver, can help manage TMAU symptoms.
Management is Possible: While there is no cure for genetic TMAU, the odor can be managed with diet, specific supplements like riboflavin, and proper hygiene.

The short answer is yes, certain vitamins can contribute to a fishy body odor, but it is not a widespread side effect and is often related to a specific metabolic condition or an overly high dosage. The key culprits are supplements containing choline or L-carnitine. When the body cannot properly break down a compound called trimethylamine (TMA), it can lead to a noticeable odor in the sweat, breath, and urine. This metabolic issue is known as trimethylaminuria (TMAU).

The Role of Choline and Carnitine

Both choline and L-carnitine are precursors to trimethylamine (TMA), the compound with the pungent, fishy smell. Choline is a nutrient essential for liver function, brain development, and nerve function. L-carnitine, often promoted as a performance-enhancing supplement, helps the body convert fat into energy. When consumed in high doses, especially via supplements, these nutrients can lead to an accumulation of TMA in the body, particularly if there is an underlying issue with metabolism.

Understanding Trimethylaminuria (TMAU)

Trimethylaminuria is a rare genetic or acquired metabolic disorder characterized by the excessive excretion of TMA. This happens due to a deficiency in the flavin-containing monooxygenase 3 (FMO3) enzyme. The FMO3 enzyme is normally responsible for converting TMA into a non-smelly compound. Individuals with a mutated FMO3 gene, or those with secondary TMAU due to other conditions, cannot process TMA efficiently. The excess TMA is then released through bodily fluids, causing a strong fishy odor.

Other Supplements and Fishy Odor

While choline and carnitine are the most common vitamin-related causes of a fishy odor, other supplements can also be a factor. For example, low-quality fish oil supplements that have gone rancid will often cause fishy burps and an unpleasant smell. Additionally, high doses of certain B vitamins, like B1 (thiamine), can cause a fishy odor in urine. It is important to distinguish the source of the smell to take appropriate action.

Supplement Types and Odor Potential

Choline Supplements: Often included in B-complex formulas or sold separately for cognitive benefits. High doses are strongly associated with fishy odor, especially in sensitive individuals.
L-Carnitine: A popular supplement for athletes and weight loss. High doses have been reported to cause a fishy smell in some users.
Fish Oil: Rancid or oxidized fish oil can produce fishy burps and a bad odor. Purchasing high-quality, fresh products can prevent this.
Other B Vitamins: While less common, certain B vitamins like B1 can alter the scent of urine.

What to Do About the Odor

If you suspect a vitamin is causing a fishy smell, the first step is to consult a healthcare professional. They can help diagnose the underlying cause and determine if trimethylaminuria is a factor.

Possible Management Steps:

Adjust Dosage: A doctor might recommend reducing the dosage of supplements containing choline or carnitine.
Dietary Modification: Limiting intake of foods rich in choline and carnitine, such as eggs, liver, and red meat, can help manage symptoms in individuals with TMAU.
Use of Supplements: Some individuals with confirmed TMAU have found relief with vitamin B2 (riboflavin) supplements, which can help increase the activity of the FMO3 enzyme.
Improve Hygiene: Using soaps with a slightly acidic pH (5.5-6.5) can help wash off the odor-causing TMA from the skin.

TMAU vs. Other Causes of Fishy Odor


Cause	Mechanism	Symptoms/Notes
Trimethylaminuria (TMAU)	Genetic or acquired deficiency of the FMO3 enzyme, leading to a buildup of trimethylamine (TMA) in the body.	Strong, persistent fishy odor from sweat, urine, and breath. Often present since childhood.
Excessive Choline/Carnitine	Overloading the body's normal metabolic pathways with high doses of TMA precursors.	Fishy odor that typically appears after consuming high-dose supplements. Decreases with reduced intake.
Rancid Fish Oil	Omega-3 fatty acids in the oil oxidize and break down, creating foul-smelling compounds.	Fishy burps and unpleasant aftertaste.
Other B Vitamins	Some B vitamins contain sulfur or can influence urine chemistry.	Can cause a strong or unusually colored urine, but usually not the distinct fishy odor associated with TMA.
Bacterial Vaginosis	Imbalance of normal bacteria in the vagina.	Fishy odor that can be confused with other causes but is localized to the vaginal area.
Kidney/Liver Disease	Impaired organ function can lead to metabolic byproducts building up and being excreted through sweat.	Systemic odor that may accompany other symptoms of organ dysfunction.

Conclusion

While a fishy smell from vitamins is not a typical occurrence, it is a possible side effect, particularly when taking high doses of supplements containing choline or L-carnitine. The most direct connection is with trimethylaminuria (TMAU), a metabolic disorder where the body cannot properly break down trimethylamine, the compound responsible for the odor. Other potential causes include rancid fish oil or other non-vitamin-related health issues. Anyone experiencing a persistent or unusual fishy odor should consult a healthcare provider for a proper diagnosis and management plan, as simply stopping the supplement may not be sufficient if an underlying condition exists. Source: National Organization for Rare Disorders (NORD).

Frequently Asked Questions

High doses of choline and L-carnitine, which are sometimes found in B-complex vitamins and standalone supplements, are the most likely culprits for a fishy body odor.

Yes, even high-quality supplements can cause a fishy odor if the dosage is too high for your body to metabolize efficiently, especially if you have an underlying metabolic disorder like TMAU.

A doctor can test for trimethylaminuria (TMAU) by analyzing a urine sample. Tracking your diet and supplement intake can also help narrow down the cause. If the smell only appears after taking a specific supplement, it's a strong indicator.

No. Rancid fish oil primarily causes fishy burps and a bad aftertaste due to oxidation, whereas TMAU causes a systemic fishy odor in sweat, urine, and breath.

While the odor itself is not dangerous, a persistent fishy smell can be a symptom of trimethylaminuria or an indicator of liver or kidney issues, which should be medically evaluated.

Yes. If the odor is caused by TMAU, avoiding or limiting foods high in choline and carnitine, such as eggs, red meat, and some legumes, can help reduce the symptoms.

Yes. Some individuals with TMAU have shown a positive response to vitamin B2 supplementation, as it can help boost the activity of the FMO3 enzyme responsible for breaking down the odor-causing compound.