Did You Know? Some people report a fishy smell after supplementing with L carnitine. This happens because certain gut bacteria convert carnitine to trimethylamine, a malodorous compound found in rotting fish. Trimethylaminuria: Definition, symptoms, and more L carnitine supplements can cause a fishy odor in the breath, sweat, and urine. This is likely due to the formation of trimethylamine (TMA). TMA is also responsible for the pungent smell of Gut Derived Metabolite, Trimethylamine N oxide (TMAO) in Cardio Metabolic Diseases: Detection, Mechanism, and Potential Therapeutics