L-carnitine is a chemical that is made in the human brain, liver, and kidneys. It helps the body turn fat into energy.

L-carnitine is important for heart and brain function, muscle movement, and many other body processes. The body can convert L-carnitine to other chemicals called acetyl-L-carnitine and propionyl-L-carnitine. But it's not clear whether the benefits of these other carnitines are the same.

Use:

Chest pain (angina). Taking L-carnitine by mouth or by IV seems to improve exercise tolerance in people with chest pain. Taking L-carnitine along with standard treatment also seems to reduce chest pain and improve exercise ability in people with cardiac syndrome X. People with this condition have chest pain but not blocked arteries.
Heart failure and fluid build up in the body (congestive heart failure or CHF). Taking L-carnitine by mouth or by IV seems to improve symptoms and increase exercise ability in people with heart failure. 
High levels of cholesterol or other fats (lipids) in the blood (hyperlipidemia). Taking L-carnitine by mouth or by IV can improve cholesterol and triglyceride levels by a small amount.
Kidney failure. The FDA has approved giving L-carnitine by IV, but not by mouth, for kidney failure. 
Conditions in a male that prevent a female partner from getting pregnant (male infertility). Taking L-carnitine by mouth, alone or together with acetyl-L-carnitine, increases sperm count and sperm movement in males with fertility problems. Some research shows that this increases the chance of pregnancy.
Swelling (inflammation) of the heart (myocarditis). Some children who have had diphtheria can develop myocarditis. Taking DL-carnitine by mouth seems to reduce the risk of myocarditis and death in these children.
A hormonal disorder that causes enlarged ovaries with cysts (polycystic ovary syndrome or PCOS). Taking L-carnitine by mouth can increase ovulation and the chance of getting pregnant in some people who don't respond to the medication clomiphene. Also, taking L-carnitine might help with weight loss and improving blood sugar levels.
Toxic side effects caused by the drug valproic acid. Toxicity caused by valproic acid seems to be linked with L-carnitine deficiency. Taking L-carnitine by mouth or by IV can prevent liver toxicity from valproic acid.

Dosage:

L-carnitine has most often been used by adults in doses of 2 grams by mouth daily. In children, L-carnitine has most often been used in doses of 50-100 mg/kg by mouth daily, for up to one year. Speak with a healthcare provider to find out what dose might be best for a specific condition.