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 Deficiency: What Is It, Causes, Symptoms, and More Osmosis Fish Odor Carnitine TikTok When a 12 month old boy in Portugal developed an unexpected and persistent smell similar to rotting fish, his parents were unsettled. The child appeared healthy, showed no signs of infection, and behaved normally.