What it is
L-carnitine is a small molecule that acts like a shuttle inside cells. Your cells burn fat for energy inside mitochondria. These are the cell’s power plants. Long fat chains cannot get into them on their own. Carnitine grabs a fat chain, carries it inside, and goes back for more. The heart and other muscles run mostly on fat. So they hold most of the body’s carnitine.
Healthy people make carnitine in the liver and kidneys. They make it from two amino acids (the building blocks of protein). People also get it from food, especially red meat. A few people have rare inherited conditions. Their bodies cannot take in carnitine or hold on to it. People on kidney dialysis can also 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
Two chemists, Gulewitsch and Krimberg, first pulled carnitine out of meat extract in 1905. Its name comes from the Latin word for flesh. Scientists in Japan worked out its chemical shape in 1927. For decades, no one knew what it did. In 1952, researchers found that “vitamin BT” is carnitine. Mealworm larvae need this in their food. In 1959, Irving Fritz showed that carnitine helps muscle and liver tissue burn fat faster.
In 1973, doctors first described people whose muscles lacked carnitine. The FDA approved the drug form, Carnitor, as tablets in 1985. A liquid to take by mouth followed in 1986. A shot followed in 1992. Leadiant Biosciences now holds these approvals. 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
For the low-carnitine conditions on its label, carnitine is a standard, proven treatment. For other uses, the results are mixed.
Male fertility: One trial in 100 men found better sperm movement on 2 g a day. But a small US trial found no effect. A larger US trial tested a supplement mix with 1 g of L-carnitine. It found no better sperm quality and no more live births.
Exercise: One study had 14 healthy men take L-carnitine with carbohydrate (starches and sugars) for 24 weeks. Their muscle carnitine went up by about a fifth. Their muscles also changed how they burned fuel.
Heart health: A 2013 study found that gut bacteria turn carnitine into a compound called TMAO. TMAO was linked to hardened arteries in mice. It was also linked to heart problems in people. No one has shown whether carnitine supplements raise the risk of heart and blood vessel disease in people.
Weight: Pooling many trials gives a different picture from the single small trial above. A 2025 review of 8 pooled analyses (16,352 people) found a small average weight loss with L-carnitine by mouth, but results varied a lot between trials. In people on kidney dialysis, pooled trials found a slightly stronger heart pump but no change in weight, blood fats or blood counts.
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.
Pooled analyses: Hamedi-Kalajahi et al. (Int J Vitam Nutr Res 2025), an umbrella meta-analysis of 8 meta-analyses (16,352 participants; 150–4000 mg/day orally for 8–30 weeks), reported effect sizes of −1.11 for body weight (95% CI −1.90 to −0.33), −0.33 for BMI and −1.34 for waist circumference, with I² of 90% for weight. Yu et al. (Nutr Metab 2026; 10 RCTs in overweight or obese women) found reductions in triglycerides (−8.01 mg/dL), total cholesterol (−7.85 mg/dL) and LDL cholesterol (−5.59 mg/dL) with no change in HDL. In haemodialysis patients, where the trials mix intravenous and oral regimens, Nabi et al. (Int Urol Nephrol 2025; 7 trials, 426 patients) found LVEF rose 1.64 percentage points (95% CI 0.10–3.17; p = 0.04) with no change in left-ventricular mass index or diastolic markers, and Vajdi et al. (BMC Nephrol 2026; 16 trials, 699 patients) found no significant effect on weight, BMI, lipids, albumin or haemoglobin. In male infertility, Niu et al. (Reprod Sci 2025; network meta-analysis of 16 studies) ranked oral L-carnitine best for progressive motility (SMD 4.19, 95% CI 1.60–10.95) and Ma et al. (Rev Int Androl 2025; 4 RCTs) reported improved concentration, morphology, motility and serum testosterone, but neither analysis reports live birth.
Regulatory picture
There are three separate facts here. First, approval. The FDA has approved levocarnitine, sold as Carnitor and as generics. It is approved for low carnitine that is inherited or caused by other inborn problems with how the body breaks down food. The shot is also approved to prevent and treat low carnitine in people on kidney dialysis. These approvals do not cover fertility, exercise or weight. Second, L-carnitine is also sold in the US as a dietary supplement. Supplements fall under food rules, not drug rules.
Third, compounding (when a pharmacy mixes its own version). Approved products are available and not in short supply. So pharmacies may not make near-copies of them. A related but different molecule, acetyl-L-carnitine, is “under evaluation” by the FDA for compounding. The FDA calls this Category 1.
In April 2026 American Regent recalled two batches of levocarnitine injection because some vials had no label. The FDA rated this a low-risk (Class III) recall.
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.
Enforcement: the only 2026 action found is a Class III recall initiated 2026-04-16 (D-0494-2026) by American Regent, Inc. of two lots (24159N0C0 and 25193N0C0) of Levocarnitine Injection USP 1 g/5 mL single-dose vials, NDC 0517-1045-01, for “Labeling: Missing Label”.
