L Carnitine Side Effects & Safety: A Complete Guide The Science of Good Health 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. Potential Adverse Effects of L Carnitine Marnys 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 Frontiers The role of trimethylamine N oxide in disease pathogenesis and the therapeutic potential of traditional Chinese medicine