What it is
L-carnitine is a small molecule that works like a shuttle inside cells. Fat is burned for energy inside mitochondria, the cell’s power plants, but long fatty-acid chains cannot cross the mitochondrial wall on their own. Carnitine latches onto them, carries them across, and goes back for more. Heart and skeletal muscle run largely on fat, so they hold most of the body’s carnitine.
Healthy people make carnitine in the liver and kidneys from two amino acids, and also take it in from food, especially red meat. A few people cannot absorb or hold on to it because of rare inherited disorders, and people on kidney dialysis can lose it during treatment. Levocarnitine is the drug name for this natural form.
Levocarnitine ((R)-3-hydroxy-4-trimethylammoniobutanoate; C7H15NO3, 161.20 g/mol) is a zwitterionic quaternary amine. Carnitine palmitoyltransferase 1 on the outer mitochondrial membrane converts long-chain acyl-CoA to acylcarnitine; carnitine–acylcarnitine translocase moves it across the inner membrane; and carnitine palmitoyltransferase 2 regenerates acyl-CoA for β-oxidation. Carnitine also buffers the acyl-CoA pool by exporting surplus acyl groups as acylcarnitines, which is the basis for the Carnitor label’s description of increased excretion of organic acids in organic acidemias. Endogenous synthesis proceeds from protein-bound lysine and methionine via γ-butyrobetaine; cellular uptake depends on the OCTN2 transporter, whose loss causes primary systemic carnitine deficiency.
Carnitor label pharmacokinetics: absolute oral bioavailability of 15.1% for tablets and 15.9% for oral solution after correction for endogenous levels, with a Cmax of about 80 µmol/L at 3.3 hours on 2 g twice daily. After a 20 mg/kg IV bolus, about 76% of the dose appeared in urine within 24 hours and the apparent terminal half-life was 17.4 hours. Carnitine that is not absorbed can be converted by gut bacteria to trimethylamine, which the liver oxidizes to TMAO.
Who made it and when
The chemists Gulewitsch and Krimberg isolated carnitine from muscle (meat) extract in 1905, and it was named after the Latin word for flesh. Its chemical structure was worked out in Japan in 1927. For decades its purpose was unclear. In 1952, researchers showed that “vitamin BT”, a factor mealworm larvae need in their diet, is carnitine. In 1959 Irving Fritz showed that carnitine speeds up the burning of fat in muscle and liver preparations.
The first human carnitine-deficiency syndrome in muscle was described in 1973. The FDA approved the drug form, Carnitor, as tablets in 1985, as an oral solution in 1986 and as an injection in 1992. Leadiant Biosciences now holds those approvals, and generic levocarnitine is also approved.
Gulewitsch and Krimberg reported carnitine from muscle extract in 1905, and Tomita and Sendju established its structure in 1927 (Hoppe-Seylers Z Physiol Chem). Fraenkel’s group had found that larvae of the mealworm Tenebrio molitor require a dietary growth factor, termed vitamin BT; Carter et al. and Bhattacharyya et al. identified it as carnitine in 1952 (Arch Biochem Biophys). Fritz reported in 1959 (Science; Am J Physiol) that carnitine stimulates long-chain fatty-acid oxidation by muscle and liver preparations. Engel and Angelini (Science 1973) described carnitine deficiency of human skeletal muscle with lipid-storage myopathy as a new syndrome.
Drugs@FDA records: Carnitor tablets (NDA 018948) approved 1985-12-27; Carnitor oral solution (NDA 019257) approved 1986-04-10; Carnitor injection (NDA 020182) approved 1992-12-16; FDA’s approval letter for supplement S-006, dated 1999-12-15 and issued to then-sponsor Sigma-Tau, added prevention and treatment of carnitine deficiency in ESRD patients on dialysis. The tablet NDA also has an efficacy supplement approved 1992-12-16. Generic tablets, oral solutions and injections are approved under ANDAs. The approved indications are limited to documented deficiency states. No compound patent covers this naturally occurring molecule; the 2018 injection label lists three Sigma-Tau method-of-use patents on treating dialysis patients (US 6,335,369, 6,429,230 and 6,696,493).
What the data say
In the deficiency conditions on its label, carnitine replacement is established treatment. Outside those conditions, the results are mixed.
Male fertility: a crossover trial in 100 men reported better sperm movement on 2 g a day, but a small US trial found no effect, and a larger US trial of a supplement mix containing 1 g of L-carnitine found no improvement in sperm quality or live births.
Exercise: a study in 14 healthy men found that 24 weeks of L-carnitine taken with carbohydrate raised muscle carnitine by about a fifth and changed how muscle burned fuel.
Heart health: a 2013 study reported that gut bacteria turn carnitine into TMAO, a compound linked to hardened arteries in mice and to heart events in people. Whether carnitine supplements raise cardiovascular risk in humans is not established.
Lenzi et al. (Fertil Steril 2003) ran a placebo-controlled crossover in 100 infertile men with reduced sperm concentration and motility, giving L-carnitine 2 g/day for 2 months, and reported significant gains in concentration and motility. Sigman et al. (Fertil Steril 2006) randomized 21 men with idiopathic asthenospermia to 2 g L-carnitine plus 1 g acetyl-L-carnitine daily or placebo for 24 weeks and found no significant change in motility or total motile count. MOXI (NCT02421887; Steiner et al., Fertil Steril 2020) randomized 171 men to a combination including 1,000 mg L-carnitine or placebo and found no improvement in semen parameters or DNA fragmentation; cumulative live birth was 15% vs 24%.
Wall et al. (J Physiol 2011) gave 2 g L-carnitine-L-tartrate plus 80 g carbohydrate twice daily for 24 weeks: muscle total carnitine rose 21%, glycogen use at 50% VO2max fell 55%, and work output rose 11% versus no change with carbohydrate alone (n=14). Koeth et al. (Nat Med 2013) showed microbiota-dependent TMAO production from L-carnitine, higher in omnivores than vegans, accelerated atherosclerosis in mice, and a link between plasma carnitine and cardiac events only with high TMAO (n=2,595). Randomized human cardiovascular outcome data are lacking.
Regulatory picture
Three separate facts. Approval: levocarnitine is FDA approved, as Carnitor and as generics, for carnitine deficiency that is inherited or caused by other inborn metabolic disorders, and the injection is also approved to prevent and treat deficiency in people on kidney dialysis. Those approvals do not extend to fertility, exercise or weight. L-carnitine is also marketed in the US as a dietary supplement, a category regulated under food law rather than drug law.
Compounding: because approved products are available and not in shortage, pharmacies may not make near-copies of them. Acetyl-L-carnitine, a related but different molecule, is listed by the FDA as “under evaluation” (Category 1) for compounding.
Approval: Carnitor tablets and oral solution are indicated for primary systemic carnitine deficiency (presenting, per the label, as Reye-like encephalopathy, hypoketotic hypoglycemia and/or cardiomyopathy) and for secondary deficiency from inborn errors of metabolism. Carnitor injection is labeled for secondary deficiency from inborn errors and for prevention and treatment of deficiency in end-stage renal disease patients undergoing dialysis. The labels state that the safety and efficacy of oral levocarnitine have not been evaluated in renal insufficiency, and that chronic high oral doses in severe renal impairment or dialysis may lead to accumulation of TMA and TMAO.
Compounding: levocarnitine is a component of FDA-approved drugs, so 503A compounding from bulk is lawful except for essentially copies of a commercially available product under section 503A(b)(1)(D). Levocarnitine was not on FDA’s drug shortage list when checked, so this article records compounding as restricted, meaning copies are not permitted rather than that all compounding is barred. Acetyl-L-carnitine and acetyl-L-carnitine hydrochloride sit in 503A Category 1 on FDA’s nominations list updated 2026-05-14; they are not on the 503A bulks list in 21 CFR 216.23.
