Carnitine is a small, water-soluble compound that the body makes from the amino acid lysine and also gets from food, especially red meat. Its best-known job is to carry long-chain fatty acids into mitochondria, where they are burned for energy. The biologically active form is L-carnitine. As a prescription drug it is called levocarnitine (brand Carnitor), and it is sold as a dietary supplement in several forms: plain L-carnitine, acetyl-L-carnitine, L-carnitine L-tartrate and propionyl-L-carnitine.
The single most important fact: levocarnitine is FDA-approved only for carnitine deficiency (inherited or caused by dialysis). In healthy people, the evidence for fat loss, endurance or recovery is modest, and most of an oral dose is not absorbed.
This page is the library’s general overview of carnitine and its forms. The L-Carnitine entry covers the same molecule. This page summarizes what the FDA labels, clinical trials and meta-analyses report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Class | Endogenous nutrient (quaternary ammonium compound); prescription drug and dietary supplement |
| Rx brand / approval | Carnitor (levocarnitine), FDA-approved 1985 (tablets, NDA 018948); oral solution and IV injection also approved; generics available |
| Supplement forms | L-carnitine, acetyl-L-carnitine (ALC), L-carnitine L-tartrate (LCLT), propionyl-L-carnitine (PLC) |
| Routes | Oral (tablet, solution); intravenous (Rx injection) |
| Labeled adult oral dose | 990 mg two or three times daily (tablets); 1–3 g/day (oral solution) |
| Oral bioavailability | ≈15% for Carnitor tablets and solution; 5–18% after 1–6 g oral doses |
| Half-life (label) | Terminal elimination half-life 17.4 h after a 20 mg/kg IV dose |
| Key mechanism | Shuttles long-chain fatty acids into mitochondria for beta-oxidation |
| US status | Rx drug (levocarnitine); other forms sold as supplements; not a controlled substance |
| WADA status | Not on the 2026 Prohibited List; IV infusions over 100 mL per 12 h are prohibited as a method |
What it is
The FDA label describes levocarnitine as “a carrier molecule in the transport of long-chain fatty acids across the inner mitochondrial membrane.” Its formula is C₇H₁₅NO₃, molecular weight 161.20 (Carnitor tablet label). Fatty acids fuel all tissues except the brain, and they are the main fuel for skeletal and cardiac muscle.
In muscle, carnitine also helps regulate the balance of acetyl-CoA to free coenzyme A inside mitochondria. Through this role it influences how the cell switches between fat and carbohydrate as fuel (Stephens et al., J Physiol 2007).
The body has three sources of carnitine (Evans and Fornasini, Clin Pharmacokinet 2003):
- diet, especially red meat;
- synthesis in the body from lysine (Koeth et al., Nat Med 2013);
- very efficient recovery by the kidneys, which reabsorb 98–99% of filtered carnitine.
The labels describe deficiency arising from genetic defects in carnitine handling, from some inborn errors of metabolism, and from losses during hemodialysis.
The main forms
| Form | What it is | Where it is used or studied |
|---|---|---|
| L-carnitine / levocarnitine | The free, natural form | Rx drug for carnitine deficiency; most weight-loss and heart trials |
| Acetyl-L-carnitine (ALC) | L-carnitine with an acetyl group attached | Depression, dementia and nerve-damage trials |
| L-carnitine L-tartrate (LCLT) | A salt of L-carnitine with tartaric acid | Exercise recovery and muscle-carnitine studies |
| Propionyl-L-carnitine (PLC) | L-carnitine with a propionyl group attached | Leg pain from peripheral artery disease (claudication) |
| D-carnitine / D,L-carnitine | The unnatural mirror-image form or a racemic mix | Older preparations; the label reports mild muscle weakness only in uremic patients given D,L-carnitine |
The forms are not interchangeable in the research. A result from an acetyl-L-carnitine depression trial says nothing about L-carnitine for fat loss, and vice versa.
Pharmacokinetics
From the Carnitor labels (tablet label; injection label):
| Parameter | Value |
|---|---|
| Absolute oral bioavailability | 15.1 ± 5.3% (tablets); 15.9 ± 4.9% (oral solution) |
| Peak level after 4 days of ≈2 g twice daily | About 80 µmol/L, at 3.3 h |
| Distribution half-life (IV) | 0.585 h |
| Terminal elimination half-life (IV, 20 mg/kg) | 17.4 h |
| Urinary excretion after IV dose | About 76% in 24 h |
| Plasma protein binding | None |
| Main metabolites (oral) | Trimethylamine N-oxide (TMAO), 8–49% of the dose; γ-butyrobetaine |
A pharmacokinetics review found that oral doses of 1–6 g have an absolute bioavailability of 5–18%. In contrast, carnitine from a normal diet may be up to 75% absorbed. Supplement doses are absorbed much less efficiently than the smaller amounts in food. The kidneys reabsorb carnitine until plasma levels reach about 40–60 µmol/L, close to normal endogenous levels. Above that threshold, extra carnitine is excreted (Evans and Fornasini 2003).
Much of the unabsorbed carnitine reaches gut bacteria, which turn it into trimethylamine. The liver then converts this to TMAO. This is the source of the TMAO concern discussed under side effects.
Getting carnitine into muscle
Muscle is the body’s slowest-equilibrating carnitine pool. Raising its content by mouth has proven difficult:
- In 8 healthy men, 2 g twice daily of L-carnitine for 3 months did not significantly raise muscle carnitine (4.10 to 4.79 µmol/g wet weight). It did not change VO₂max or peak power (Wächter et al., Clin Chim Acta 2002).
- In a 24-week randomized double-blind trial in 14 healthy men, 2 g of LCLT taken with 80 g of carbohydrate twice daily raised muscle total carnitine by 21%. The carbohydrate was there to raise insulin, which drives carnitine into muscle. The control group took carbohydrate alone (Wall et al., J Physiol 2011).
So the evidence suggests muscle carnitine rises only slowly, over months, and possibly only when taken with enough carbohydrate to raise insulin.
Approved medical use
Levocarnitine has been approved in the US since 1985 (Drugs@FDA, NDA 018948). The labeled indications are:
- Primary systemic carnitine deficiency (oral). The label describes recurrent Reye-like brain dysfunction, low blood sugar without ketones, and/or heart muscle disease (cardiomyopathy).
- Secondary carnitine deficiency from inborn errors of metabolism (oral and IV). Examples include propionic acidemia, methylmalonic aciduria and medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. Carnitine binds the toxic acyl compounds that build up, and the resulting acylcarnitines are excreted in urine.
- Carnitine deficiency in end-stage renal disease (ESRD) on dialysis (IV only). Dialysis removes carnitine, and these patients also make less and eat less.
| Product | Population | Labeled dose |
|---|---|---|
| Tablets (330 mg) | Adults | 990 mg two or three times daily |
| Tablets or oral solution | Infants and children | Start at 50 mg/kg/day; 50–100 mg/kg/day in divided doses, maximum 3 g/day |
| Oral solution (1 g/10 mL) | Adults | Start at 1 g/day; 1–3 g/day for a 50 kg person |
| IV injection | Metabolic disorders | 50 mg/kg by slow bolus or infusion |
| IV injection | ESRD on hemodialysis | 10–20 mg/kg dry weight after each dialysis session, adjusted by pre-dialysis carnitine levels |
The label says oral doses should be spaced through the day and taken slowly with or after meals to reduce stomach upset.
What the research shows
Weight and fat loss (L-carnitine)
Two meta-analyses of randomized trials found a small effect on body weight:
| Meta-analysis | Trials (n) | Body weight | Other outcomes |
|---|---|---|---|
| Pooyandjoo et al., Obes Rev 2016 | 9 RCTs (911) | −1.33 kg (95% CI −2.09 to −0.57) | BMI −0.47 kg/m²; the effect shrank with longer use |
| Talenezhad et al., Clin Nutr ESPEN 2020 | 37 RCTs (2,292) | −1.21 kg (95% CI −1.73 to −0.68) | Fat mass −2.08 kg; no effect on waist circumference or body fat % |
The larger analysis found the effect on weight was greatest around 2,000 mg/day and confirmed only the body weight result in high-quality trials. A loss of about 1 kg over weeks to months is small, and the Pooyandjoo analysis found the effect faded over time.
Muscle carnitine and exercise performance
In the Wall trial above, the LCLT plus carbohydrate group used 55% less muscle glycogen during low-intensity cycling. They also produced less lactate during high-intensity cycling, and increased work output in a 30-minute performance trial by 11% from baseline. The control group showed no change (Wall et al. 2011). This was a small study (n = 14) that required 160 g of extra carbohydrate a day.
The 3-month Wächter study, which used L-carnitine without the carbohydrate load, found no significant change in muscle carnitine or performance. Together, these studies do not support claims that carnitine quickly increases fat burning during exercise.
Recovery and muscle damage (mostly LCLT)
In a crossover study, 10 resistance-trained men took LCLT (2 g L-carnitine/day) or placebo for 3 weeks, then did a high-repetition squat session. Compared with placebo, LCLT reduced blood markers of muscle damage such as myoglobin and creatine kinase. On MRI, the area of muscle disruption was 41–45% of the placebo area (Volek et al., Am J Physiol Endocrinol Metab 2002).
A meta-analysis of 7 RCTs found that L-carnitine reduced muscle soreness at 0–96 hours after exercise. It lowered creatine kinase, myoglobin and lactate dehydrogenase only at the 24-hour time point (Yarizadh et al., J Am Coll Nutr 2020). The authors noted the benefit was seen mainly in resistance-training studies and untrained people.
Androgen receptors and hormones
The same research group reported that 3 weeks of LCLT (2 g L-carnitine/day) in 10 resistance-trained men raised muscle androgen receptor content before exercise (12.9 vs 11.2 arbitrary units with placebo) (Kraemer et al., Med Sci Sports Exerc 2006). This was a small, short study measuring a marker. It did not show any increase in testosterone, muscle mass or strength. It should not be read as evidence that carnitine has an anabolic effect.
Depression (acetyl-L-carnitine)
A meta-analysis of 12 RCTs (791 participants) found that ALC reduced depressive symptoms compared with placebo or no treatment (standardized mean difference −1.10, 95% CI −1.65 to −0.56). In 3 trials, ALC performed similarly to established antidepressants (Veronese et al., Psychosom Med 2018). Heterogeneity was very high (I² = 86%), the trials were small, and the authors called for large trials to confirm or refute the results.
Dementia and cognition (acetyl-L-carnitine)
A Cochrane review of 11 trials in Alzheimer’s disease found a benefit on clinicians’ global impression at 12 and 24 weeks, but not at 52 weeks. It found no evidence of benefit on cognition, dementia severity or daily function (Hudson and Tabet, Cochrane 2003).
Chemotherapy nerve damage (acetyl-L-carnitine): harm signal
ALC was tested to prevent nerve damage from taxane chemotherapy in the SWOG S0715 trial. In this randomized double-blind trial, 409 women with breast cancer took ALC 1,000 mg three times daily or placebo for 24 weeks. ALC did not prevent neuropathy. Instead it made neuropathy worse at 24 weeks, and grade 3–4 neurotoxicity occurred in 8 vs 1 patients (Hershman et al., J Clin Oncol 2013). The worse scores persisted over 2 years of follow-up (Hershman et al., J Natl Cancer Inst 2018).
Claudication (propionyl-L-carnitine)
In a 6-month double-blind trial in 155 patients with claudication, PLC 2 g/day increased peak walking time by 54%, vs 25% on placebo (Hiatt et al., Am J Med 2001). A 2021 Cochrane review of 12 studies (1,423 participants) found modest gains. Maximal walking distance improved by about 51 m more than placebo, and pain-free walking distance by about 33 m. The certainty of evidence was moderate, and a high proportion of the studies were backed by drug companies (Kamoen et al., Cochrane 2021).
Heart attack (L-carnitine)
A 2013 meta-analysis of 13 controlled trials (3,629 patients) after an acute heart attack reported lower all-cause mortality with L-carnitine (risk ratio 0.78, 95% CI 0.60–1.00). It also found fewer ventricular arrhythmias and less angina, with no effect on heart failure or repeat heart attack (DiNicolantonio et al., Mayo Clin Proc 2013). The mortality result was borderline, the trials largely predate modern heart-attack care, and the authors called for large new trials. Carnitine is not part of standard heart-attack treatment.
Side effects and risks
Common effects
The label lists mild gastrointestinal effects with oral L- or D,L-carnitine: nausea, vomiting, abdominal cramps and diarrhea. The label also mentions drug-related body odor and says lowering the dose often reduces both (Carnitor tablet label).
Seizures and allergic reactions
- Seizures have been reported with oral or IV levocarnitine, in people with or without a seizure history. In people with existing seizures, more frequent or severe seizures have been reported.
- Hypersensitivity: rash, hives and facial swelling with oral use; anaphylaxis, throat swelling and bronchospasm with IV use, mostly in dialysis patients (injection label).
TMAO and heart risk
Gut bacteria convert unabsorbed carnitine into trimethylamine, which the liver turns into TMAO. In mice, carnitine-rich diets raised TMAO and sped up atherosclerosis (plaque buildup in arteries). In 2,595 people undergoing heart evaluation, higher plasma carnitine predicted heart disease and major cardiac events, but only in those who also had high TMAO. Omnivores made more TMAO from a carnitine dose than vegans or vegetarians (Koeth et al., Nat Med 2013).
In older women, 1,500 mg/day of LCLT for 24 weeks raised plasma TMAO compared with placebo (Olek et al., Oxid Med Cell Longev 2019). TMAO returned to normal within 4 months of stopping, with no changes in blood lipids during follow-up (Samulak et al., Nutrients 2019). No trial has shown that carnitine supplements cause heart attacks or strokes in people. The TMAO link is an association and an animal finding, not proven harm.
Kidney disease
The label warns that long-term high oral doses in people with severe kidney impairment or on dialysis may cause buildup of trimethylamine and TMAO, which are normally excreted in urine. The label also states that the safety and efficacy of oral levocarnitine have not been evaluated in renal insufficiency. The approved product for dialysis-related deficiency is the IV injection.
Interactions
- Warfarin: the label reports increased INR (a clotting-time measure). It recommends monitoring INR after starting levocarnitine or changing the dose.
- Taxane chemotherapy: ALC worsened nerve damage in SWOG S0715, as described above.
Pregnancy and breastfeeding
Animal studies in rats and rabbits at up to 3.8 times the human dose showed no harm, but there are no adequate studies in pregnant women. The label says to use it in pregnancy only if clearly needed. It has not been studied in breastfeeding.
Legal status and use in sport
- US: levocarnitine is a prescription drug (Carnitor and generics; Drugs@FDA). L-carnitine and its other forms are also widely sold as dietary supplements. Carnitine is not a controlled substance.
- WADA: carnitine itself is not listed. However, the list prohibits “intravenous infusions and/or injections of more than a total of 100 mL per 12-hour period,” except those legitimately received in hospital treatment, surgery or clinical diagnostic investigations. This rule applies to any substance given this way, carnitine included (WADA 2026 Prohibited List, M2.2).
Key takeaways
- One molecule, several forms. L-carnitine (levocarnitine), acetyl-L-carnitine, L-carnitine L-tartrate and propionyl-L-carnitine are each studied for different uses. Results from one form do not transfer to another.
- Approved only for deficiency. The FDA approves levocarnitine for primary carnitine deficiency, some inborn errors of metabolism, and (IV) dialysis-related deficiency.
- Poorly absorbed in supplement doses. About 5–18% of a 1–6 g oral dose is absorbed. Raising muscle carnitine took 24 weeks plus a large daily carbohydrate load in the one positive trial.
- Modest benefits in healthy people. Meta-analyses show about 1.2–1.3 kg extra weight loss and less post-exercise soreness. Strong performance or anabolic effects have not been shown.
- Real risks exist. Acetyl-L-carnitine worsened chemotherapy nerve damage in a large trial. Supplements raise TMAO, a marker linked to heart disease. The label also lists seizures, a warfarin interaction and metabolite buildup in kidney failure.
Sources
- Leadiant Biosciences. Carnitor (levocarnitine) tablets, oral solution and sugar-free oral solution, prescribing information. DailyMed.
- Leadiant Biosciences. Carnitor (levocarnitine) injection, prescribing information. DailyMed.
- US Food and Drug Administration. Drugs@FDA: Carnitor, NDA 018948. FDA.
- Evans AM, Fornasini G. Pharmacokinetics of L-carnitine. Clin Pharmacokinet. 2003;42(11):941–967. PubMed.
- Stephens FB, Constantin-Teodosiu D, Greenhaff PL. New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle. J Physiol. 2007;581(Pt 2):431–444. PubMed.
- Koeth RA, Wang Z, Levison BS, et al. Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat Med. 2013;19(5):576–585. PubMed.
- Wächter S, Vogt M, Kreis R, et al. Long-term administration of L-carnitine to humans: effect on skeletal muscle carnitine content and physical performance. Clin Chim Acta. 2002;318(1-2):51–61. PubMed.
- Wall BT, Stephens FB, Constantin-Teodosiu D, et al. Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. J Physiol. 2011;589(Pt 4):963–973. PubMed.
- Pooyandjoo M, Nouhi M, Shab-Bidar S, et al. The effect of (L-)carnitine on weight loss in adults: a systematic review and meta-analysis of randomized controlled trials. Obes Rev. 2016;17(10):970–976. PubMed.
- Talenezhad N, Mohammadi M, Ramezani-Jolfaie N, et al. Effects of l-carnitine supplementation on weight loss and body composition: a systematic review and meta-analysis of 37 randomized controlled clinical trials with dose-response analysis. Clin Nutr ESPEN. 2020;37:9–23. PubMed.
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- Yarizadh H, Shab-Bidar S, Zamani B, et al. The effect of L-carnitine supplementation on exercise-induced muscle damage: a systematic review and meta-analysis of randomized clinical trials. J Am Coll Nutr. 2020;39(5):457–468. PubMed.
- Kraemer WJ, Spiering BA, Volek JS, et al. Androgenic responses to resistance exercise: effects of feeding and L-carnitine. Med Sci Sports Exerc. 2006;38(7):1288–1296. PubMed.
- Veronese N, Stubbs B, Solmi M, et al. Acetyl-L-carnitine supplementation and the treatment of depressive symptoms: a systematic review and meta-analysis. Psychosom Med. 2018;80(2):154–159. PubMed.
- Hudson S, Tabet N. Acetyl-L-carnitine for dementia. Cochrane Database Syst Rev. 2003;(2):CD003158. PubMed.
- Hershman DL, Unger JM, Crew KD, et al. Randomized double-blind placebo-controlled trial of acetyl-L-carnitine for the prevention of taxane-induced neuropathy in women undergoing adjuvant breast cancer therapy. J Clin Oncol. 2013;31(20):2627–2633. PubMed.
- Hershman DL, Unger JM, Crew KD, et al. Two-year trends of taxane-induced neuropathy in women enrolled in a randomized trial of acetyl-L-carnitine (SWOG S0715). J Natl Cancer Inst. 2018;110(6):669–676. PubMed.
- Hiatt WR, Regensteiner JG, Creager MA, et al. Propionyl-L-carnitine improves exercise performance and functional status in patients with claudication. Am J Med. 2001;110(8):616–622. PubMed.
- Kamoen V, Vander Stichele R, Campens L, et al. Propionyl-L-carnitine for intermittent claudication. Cochrane Database Syst Rev. 2021;12(12):CD010117. PubMed.
- DiNicolantonio JJ, Lavie CJ, Fares H, et al. L-carnitine in the secondary prevention of cardiovascular disease: systematic review and meta-analysis. Mayo Clin Proc. 2013;88(6):544–551. PubMed.
- Olek RA, Samulak JJ, Sawicka AK, et al. Increased trimethylamine N-oxide is not associated with oxidative stress markers in healthy aged women. Oxid Med Cell Longev. 2019;2019:6247169. PubMed.
- Samulak JJ, Sawicka AK, Samborowska E, Olek RA. Plasma trimethylamine-N-oxide following cessation of L-carnitine supplementation in healthy aged women. Nutrients. 2019;11(6):1322. PubMed.
- World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.