How Long Should You Take L Carnitine for Weight Loss Results? Should I use L Carnitine on rest days or just before workouts? TrimRX When to Take L Carnitine for Weight Loss: A Pro Timing Guide Supplements don't replace the workbut they can support it L Carnitine helps your body turn fat into usable energy, so you can push harder and recover better. Consistency first. Supplements second. Results