L-Carnitine

Origin and structure

L-carnitine is not a peptide. Peptides are chains of amino acids; L-carnitine is a single small molecule (C7H15NO3, molecular weight 161.20) built around a nitrogen atom that carries three methyl groups and a permanent positive charge, a so-called quaternary ammonium compound. The body makes it mainly in the liver and kidneys from two amino acids, lysine and methionine, and the diet adds more, especially red meat. As a medicine it is called levocarnitine.

Only the L-form is biologically active. The US label notes that mild muscle weakness was described only in kidney patients given DL-carnitine, a mix of the L- and D-forms. Acetyl-L-carnitine and propionyl-L-carnitine are related esters with their own, separate research; this article is about L-carnitine itself.

How it works

Long-chain fatty acids cannot cross the inner membrane of the mitochondria on their own. L-carnitine binds them as acylcarnitines, which are carried across and released inside, where the fatty acids are broken down to make ATP. Skeletal muscle and heart muscle run mainly on fat, so they depend heavily on this shuttle. It also works the other way: carnitine binds excess or toxic acyl groups that build up in some inherited metabolic diseases, and the resulting acylcarnitines are quickly excreted in urine. That is why it is used in disorders such as propionic acidaemia, methylmalonic aciduria and MCAD deficiency.

Supplements are poorly absorbed. In the studies behind the Carnitor label about 15% of an oral dose reached the blood; the Dutch product information says under 10%, and very variable. After an intravenous dose the terminal half-life was about 17 hours, and about 76% of the dose left in the urine within 24 hours. Gut bacteria break down much of what is not absorbed into trimethylamine, which the body turns into trimethylamine N-oxide (TMAO).

Raising carnitine in muscle is harder than raising it in blood. In a 24-week study, muscle carnitine rose by 21% in healthy men who received L-carnitine twice a day together with carbohydrate, given to raise insulin, which helps muscle take carnitine up.

Is L-carnitine approved?

Yes, as a prescription medicine for carnitine deficiency, in the Netherlands and in the US. It is also sold as a food supplement, which is a food, not a medicine.

Research-grade L-carnitine, sold for laboratory use, is neither. It has no marketing authorisation as a medicine and may not be used in people. The approvals below describe the licensed products, not the research material.

• EU: there is no EU-wide authorisation through the EMA; levocarnitine is authorised nationally. In the Netherlands the CBG registered Carnitene injection (200 mg/mL) and oral solution (100 mg/mL) on 3 February 1986 and Carnitene 330 mg tablets on 27 January 1987. The register also lists Nefrocarnit, Carneus, generic tablets and parallel imports, all prescription-only. The tablets and oral solution are licensed for primary (systemic) carnitine deficiency; the injection for primary deficiency only in acute cases when oral treatment is not possible, and for carnitine deficiency in people on haemodialysis.
• US: Carnitor tablets were approved in December 1985 as a new molecular entity, the oral solution in April 1986 and the injection in December 1992. In December 1999 the injection was also approved to prevent and treat carnitine deficiency in end-stage kidney disease on dialysis. Today the tablets and oral solution are labelled for primary systemic carnitine deficiency and for secondary deficiency caused by inborn errors of metabolism; generic levocarnitine is also approved.
• Dialysis: the US label says that levocarnitine raises carnitine levels in people on dialysis, but that its effect on symptoms and on clinical outcomes in this group has not been determined. The Dutch label asks for the response to be reviewed every 3 months and treatment to be stopped if there is no clinical improvement within 9 to 12 months.

What the research shows

For carnitine deficiency the case is clear enough for regulators: the US label notes that in primary deficiency, carnitine rapidly relieved signs and symptoms in some patients, particularly those with heart muscle disease. Outside deficiency, most human studies are small, short or limited to specific patient groups, and the effects are modest or mixed. The main findings:

• Haemodialysis: in a 9-week study, 12 dialysis patients received 20 mg/kg intravenously three times a week after dialysis; their pre-dialysis carnitine level rose from about 21 to about 190 µmol/L after 8 weeks (normal 40 to 50). Whether that improves symptoms or outcomes has not been shown.
• Weight (Talenezhad, Clin Nutr ESPEN 2020): a meta-analysis of 37 randomised trials with 2,292 participants found 1.21 kg more weight loss than control (95% CI 0.68 to 1.73), 0.24 kg/m² lower BMI and 2.08 kg less fat mass, but no change in waist circumference or body-fat percentage. The effect appeared largest at about 2 g a day.
• After a heart attack (DiNicolantonio, Mayo Clin Proc 2013): a meta-analysis of 13 controlled trials with 3,629 people linked L-carnitine to 27% lower all-cause mortality (odds ratio 0.73, 95% CI 0.54 to 0.99), fewer ventricular arrhythmias and less angina, but not to less heart failure or fewer repeat infarctions. The authors called for large, modern trials.
• CEDIM 2 (Cardiology 2006), the largest single trial: 2,330 people with an anterior heart attack, stopped before the planned 4,000 because enrolment was slow. Death or heart failure at 6 months occurred in 9.2% on L-carnitine against 10.5% on placebo, not a significant difference; death within 5 days, a secondary endpoint, was lower (hazard ratio 0.61).
• Cancer-related fatigue (Cruciani, J Clin Oncol 2012): in an ECOG phase 3 trial, 376 people with cancer and fatigue received 2 g a day or placebo for 4 weeks. Fatigue improved in both groups, with no difference between them, also among those who were carnitine-deficient at the start.
• Exercise (Wall, J Physiol 2011): in a randomised, double-blind study of 14 healthy men, 24 weeks of L-carnitine with carbohydrate raised muscle carnitine by 21%, and during light cycling the carnitine group used 55% less muscle glycogen than the carbohydrate-only group; work output in a 30-minute performance test rose by 11% from baseline, while the carbohydrate-only group did not change. It is one small study.
• Heart risk through the gut (Koeth, Nat Med 2013): gut bacteria turned L-carnitine into TMAO; in mice long-term L-carnitine sped up atherosclerosis, and among 2,595 people undergoing cardiac evaluation, a high blood carnitine level predicted heart disease and major cardiac events (heart attack, stroke or death) only when TMAO was also high. These are animal and observational data, not proof that supplements cause harm.

Side effects and warnings

The Dutch product information lists nausea, vomiting, abdominal pain and diarrhoea, the last mainly after high oral doses, as common (1 to 10 in 100 people). Less common (1 to 10 in 1,000) are headache, a changed sense of taste, indigestion, dry mouth, high or low blood pressure, muscle cramps and an unusual body odour. A fishy smell is linked to trimethylamine; the US label notes that lowering the dose often reduces body odour and stomach complaints.

Serious reactions are rare. Seizures have been reported with oral and intravenous use, in people with and without epilepsy, and in people with epilepsy their frequency or severity can increase. Serious allergic reactions, including anaphylaxis, swelling of the larynx and bronchospasm, have been reported, mostly after intravenous doses in dialysis patients and sometimes within minutes; rash, hives and facial swelling have been reported with oral levocarnitine.

Very rarely the INR rises in people on coumarin anticoagulants such as warfarin, so the labels advise checking it. In people with diabetes on insulin or glucose-lowering tablets it can cause low blood sugar. Long-term high oral doses are not advised in severe kidney failure or on dialysis, because trimethylamine and TMAO build up. Medicines that increase the loss of carnitine in urine, such as valproic acid, pivalate-containing prodrugs, zidovudine, cephalosporins, cisplatin, carboplatin and ifosfamide, can lower its availability. The Dutch product information advises against use in pregnancy as a precaution; the US label says to use it in pregnancy only if clearly needed. No toxicity from overdose has been reported; large doses cause diarrhoea.

Frequently asked questions

Is L-carnitine a peptide?

No. Peptides are chains of amino acids; L-carnitine is a single small molecule with a molecular weight of 161 that the body builds from two amino acids, lysine and methionine. Chemically it is a quaternary ammonium compound, a vitamin-like nutrient rather than a peptide. It has no amino-acid sequence.

Does L-carnitine help with weight loss?

A little, on average. A 2020 meta-analysis of 37 randomised trials with 2,292 participants found about 1.2 kg more weight loss and 2.1 kg less fat mass than control, with no change in waist circumference or body-fat percentage; the effect looked largest at about 2 g a day. When only the higher-quality trials were analysed, just the effect on body weight held up. That is far smaller than with medicines licensed for weight loss.

Is L-carnitine legal in the Netherlands?

Yes. As a medicine, levocarnitine (Carnitene and other brands and generics) is registered in the Netherlands and available only on prescription, for carnitine deficiency. L-carnitine is also sold as a food supplement, which counts as a food, not a medicine. Research-grade L-carnitine is neither: it is sold for laboratory research and may not be used in people.

Is L-carnitine banned in sport?

The substance is not: L-carnitine does not appear on the WADA Prohibited List for 2026. The method can be. Under rule M2.2, intravenous infusions and/or injections of more than 100 mL in total within 12 hours are prohibited at all times, unless legitimately received during hospital treatment, surgical procedures or clinical diagnostic investigations. The limit applies to the volume, whatever the drip contains. The Dopingautoriteit publishes the Dutch translation of the list.

Does L-carnitine raise TMAO, and is that bad for the heart?

Gut bacteria turn L-carnitine that is not absorbed into trimethylamine, which the body converts into TMAO. In a 2013 study, long-term L-carnitine sped up atherosclerosis in mice through this route, and in people a high blood carnitine level predicted heart problems only when TMAO was also high. That is animal and observational evidence. Trials after a heart attack, by contrast, point to no harm and possibly fewer deaths, so the question is open.

Is research-grade L-carnitine the same as Carnitene or Carnitor?

No. Carnitene and Carnitor are licensed medicines with a known content per tablet, bottle or ampoule, made and tested under pharmaceutical rules; their injections contain 200 mg/mL and are given only into a vein. Research-grade L-carnitine is sold for laboratory use; its identity and purity are what a certificate of analysis says, nothing more, and it may not be used in people.

Storage and handling

Store according to product-label instructions and batch handling notes. Lyophilized research peptides are commonly kept dry, protected from light, and handled with clean laboratory technique.

References

Checked against these sources on 28 October 2026.