Although only a doctor can determine how much L-carnitine is right for you -- or if you should take the supplement at all -- adult doses typically vary from 1 to 3 grams per day, according to the University of Maryland Medical Center. Taking more than 5 grams per day can result in side effects such as diarrhea and rash. UMMC also warns that L-carnitine may not be safe in any dosage if you're taking certain medications, such as thyroid hormones.
L-carnitine, a derivative of the amino acid lysine, occurs naturally in meat, fish, dairy and some plant foods, such as avocado. The body also produces this nutrient on its own. Therefore, the Linus Pauling Institute says that most people, including vegetarians, are unlikely to have a deficiency that requires supplementation. The institute notes that L-carnitine supplements may help relieve medical conditions such as peripheral arterial disease and Alzheimer's -- but only your doctor can make that call.