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 Is L Carnitine Safe? Watch FIRST BEFORE Taking L Carnitine Supplements L Carnitine Linus Pauling Institute Oregon State University 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.