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 Exploring the Efficacy of Carnitine as a Dietary Supplement Acetyl L Carnitine and Erectile Dysfunction: What the Evidence Shows iSatori Liquid L Carnitine LS3 3000 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.