L-carnitine is one form of carnitine, which helps your body make energy. Some people take it as a supplement to boost athletic performance, even though there is no proof that it helps. L-carnitine supplements can sometimes cause nausea, diarrhea, cramps, or vomiting. High doses can make you smell "fishy." Doctors don’t recommend it for women who are pregnant, because of lack of evidence about its safety.