Optimizing endurance performance in elite swimmers: effects of L carnitine supplementation and hydration strategies Applied Water Science Springer Nature Link 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 L Carnitine Uses, Benefits, and Side Effects thegoodbug L Carnitine Injection Levocarnitine IV Carnitine HealthyPlace 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.