When to take L carnitine: Dosage, forms, and side effects l Carnitine Supplementation in Recovery after Exercise Comparison of short term and long term effects of peroral L carnitine intake: clinical implications of elevated TMAO levels in cardiovascular complications 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.