L Carnitine: Benefits, Side Effects, FAQ, Sources, and Dosage When to take L carnitine: Dosage, forms, and side effects L carnitine helps transport fatty acids into your cells' mitochondria, where they can be used for energy. That's why it's often marketed for fat loss and energy support. Common use: People usually take Amazon.com: Cutler Nutrition Liquid L Carnitine Fat Burner Supplement with Acetyl L Carnitine & L Carnitine Tartrate Fat Burner Formula for Optimal Absorption, Energy & Muscle Support Sour Gummy Flavor : Health &