L Carnitine as Pre Workout Supplement: Guide The Science of Good Health Oxyenergys L Carnitine. My go to pre post workout supplement. Pre workout I usually take it before my workout session for energy production. Post workout Ill take it the day after my workout When Should You Take L Carnitine? MuscleTech Should I use L Carnitine on rest days or just before workouts? L Carnitine for Fat Loss: The Science Backed Secret to Shredding Safely