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 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. Urine smells like fish: Causes and treatment 11 Urine Smells Explained In Everyday Life Some food and drink habits show up in a very ordinary place, and people are always a little surprised by how fast it happens. A L Carnitine: Benefits, side effects, and dosage The Role of L Carnitine in Kidney Disease and Related Metabolic Dysfunctions