Trimethylaminuria, or fish odour syndrome is an uncommon condition that causes an unpleasant, fishy smell. The body is unable to turn a strong smelling chemical called trimethylamine produced in the gut when L Carnitine: Benefits, Uses, and Side Effects Lifespan Research Institute 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. Does L Carnitine Work for Fat Loss? A Dietitian Explains