Carnitine's effectiveness depends on the reason you are taking it. Most people can make all the carnitine they need using amino acids found in food, unless they eat no meat or dairy. But some people have genetic conditions or medical problems that interfere with the body's ability to convert food into carnitine; these people may need supplements. Large doses may also help treat certain medical conditions.
L-carnitine is rated by the NMCD as "possibly effective" for angina (pain associated with coronary heart disease), congestive heart failure, symptoms of an overactive thyroid, infertility, low birth weight in premature infants and heart attack. It does not improve athletic performance or endurance, contrary to claims by some manufacturers.
Acetyl-L-carnitine is "possibly effective" for age-related cognitive impairment, age-related testosterone deficiency, Alzheimer's disease, diabetic neuropathy and some forms of infertility in men.
Propionyl-L-carnitine is also considered "possibly effective" for treating angina, congestive heart failure and erectile dysfunction.