L Carnitine: What it's used for, benefits and how to take it correctly Justloading Carnitine Blend Rootcology 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. L Carnitine: Benefits, Uses, Side Effects, 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 Carnitine: Genetic Variants Affecting Mitochondrial Energy and Health