In 30 seconds
- L-carnitine is well tolerated at the usual doses: 3 g a day of tartrate for 3 weeks did not change any liver, kidney or blood marker compared with placebo.
- Digestive discomfort and a fishy body odour are described at about 3 g a day, according to the US National Institutes of Health.
- In a 5-week trial with 73 adults aged 21 to 65, adverse effects were mild and as frequent with L-carnitine as with placebo.
- If you take anticoagulants or anti-epileptic drugs, have a thyroid or kidney problem, or are pregnant, check with your doctor first.
L-carnitine is well tolerated at the usual doses. The side effects described, mainly digestive discomfort and a fishy body odour, appear at about 3 g a day 1. If you take certain medicines or have a specific health condition, check with your doctor first.
The safety trials
What was seen when taking it every day
Is L-carnitine safe?
To begin with, it is not a foreign substance to your body. You make it yourself in the liver, kidneys and brain from two amino acids, lysine and methionine 1. And you also get it from meat.
As a supplement, one of the most studied forms in sport is L-carnitine tartrate. In one trial, 10 healthy men took 3 g a day for 3 weeks and placebo for another 3 weeks. They had a full panel of blood, liver, kidney and mineral tests: no value changed compared with placebo 3.
In a 5-week trial with 73 adults aged 21 to 65, adverse effects were mild and as frequent with L-carnitine as with placebo, and none was serious 4.
Does it affect the liver or kidneys?
It is a common question. In the safety trial with tartrate, liver and kidney markers did not change after 3 weeks at 3 g a day 3.
The precautions are for people who already have kidney disease. Muscle weakness has been described in people with uraemia, and many people on haemodialysis actually have low carnitine 1. In those cases, their doctor decides.
L-carnitine a day
Where discomfort is described
Possible side effects
According to the US NIH Office of Dietary Supplements, at about 3 g a day nausea, vomiting, stomach cramps, diarrhoea and a fishy body odour may appear 1. Other centres also list heartburn and headache among the possible effects 2.
| Effect | When it is described | What to do |
|---|---|---|
| Nausea, vomiting or stomach cramps | Mainly at about 3 g a day | Stay within the dose on the label |
| Diarrhoea | Mainly at about 3 g a day | Stay within the dose on the label |
| Fishy body odour | At high doses, of about 3 g a day | Go back to the dose on the label |
| Heartburn or headache | Among the possible effects described | If it does not go away, check with your doctor |
Sources: US NIH Office of Dietary Supplements and Memorial Sloan Kettering Cancer Center.
The US NIH has not set an upper intake limit 1, so the practical reference is the 3 g at which discomfort is described. The golden rule is simple: follow the dose on the label and do not double it.
And the fishy smell?
It is the most striking effect, and the one that raises the most questions. It is described at high doses, of about 3 g a day 1. If you ever notice it, the solution is to go back to the recommended dose.
What TMAO is
You may have read something about TMAO. It is a substance that gut bacteria make from the carnitine that is not absorbed. It has been linked to a higher cardiovascular risk, and people who eat meat produce more of it than vegetarians 1.
The NIH sum it up cautiously: the implications of these findings are not yet well understood and more research is needed 1. If you have cardiovascular disease, talk to your doctor before taking any supplement.
Who should check first
For some people, it is best to ask first. Check with your doctor if:
- You take anticoagulants, such as warfarin or acenocoumarol: L-carnitine could add to the anticoagulant effect 2.
- You have epilepsy or take anti-epileptic drugs, such as valproic acid. Seizures have been described in people with epilepsy, and these drugs lower the body's carnitine 1.
- You have a thyroid problem or take thyroid hormone: in one trial, carnitine slowed the action of thyroid hormones 2.
- You have kidney disease: muscle weakness has been described in people with uraemia 1.
- You are pregnant or breastfeeding, or it is for a child.
If any of these applies to you, a quick word with your doctor will put your mind at rest.
How to take it properly
- 20 ml a day, 30 minutes before training. On rest days, with a meal.
- Within the dose. Discomfort is described at higher amounts 1. The dose used in studies is in how much L-carnitine per day.
- Consistently. Recovery studies measured its effects after 3 to 5 weeks of daily use 4.
- Once opened, use it within 30 days. With one serving a day, the bottle lasts 25 days.
L-carnitine contains no caffeine or other stimulants, so it also fits in if you train in the evening. It is one of the reasons many people prefer it to fat burners with stimulants, which we compare in fat burners: which ones work. If you are unsure about the timing, we explain it in L-carnitine: before or after training?.
What you can expect
Taken properly, L-carnitine has interesting studies behind it:
- Weight: in a meta-analysis of 9 trials with 911 adults, those who took it lost 1.33 kg more than the control group 6.
- Muscle soreness: in a meta-analysis of 14 trials, with 1-3 g a day there was less muscle soreness 24 and 48 hours after exercise 5. There were also fewer markers of muscle damage.
- Recovery: in the 5-week trial, with 2 g a day as tartrate, creatine kinase, a marker of muscle damage, was 28% higher in the placebo group 4. And strength was better preserved.
You will find all the studies in L-carnitine: what the studies say and in L-carnitine tartrate and recovery, and the practical summary in what it does and how to take it.
Frequently asked questions
Does L-carnitine have side effects?
It can, mainly at high doses: at about 3 g a day, nausea, digestive discomfort, diarrhoea and a fishy body odour are described. At the usual doses it is well tolerated: in a 5-week trial, adverse effects were mild and as frequent as with placebo.
Does L-carnitine make you smell of fish?
It can at high doses, of about 3 g a day, according to the US National Institutes of Health. If it happens to you, go back to the dose on the label.
Who should not take L-carnitine?
Check with your doctor first if you take anticoagulants or anti-epileptic drugs, have epilepsy, kidney disease or a thyroid problem, or are pregnant or breastfeeding. It is also a supplement for adults, not for children.
Is L-carnitine bad for your heart?
There is no evidence that it is in healthy people. Researchers are studying TMAO, a substance gut bacteria make from carnitine, but they say themselves that its implications are not yet well understood. If you have cardiovascular disease, check with your doctor.
Can you take L-carnitine every day?
Yes. Recovery studies give it daily for 3 to 5 weeks. Just stay within the dose.
Sources
- Office of Dietary Supplements, National Institutes of Health. Carnitine: fact sheet for health professionals. View publication
- Memorial Sloan Kettering Cancer Center. Carnitine. About Herbs, Botanicals and Other Products. Updated 15 October 2026. View publication
- Rubin MR, Volek JS, Gómez AL, et al. Safety measures of L-carnitine L-tartrate supplementation in healthy men. J Strength Cond Res. 2001;15(4):486-490. View publication
- Stefan M, Sharp M, Gheith R, et al. L-carnitine tartrate supplementation for 5 weeks improves exercise recovery in men and women: a randomized, double-blind, placebo-controlled trial. Nutrients. 2021;13(10):3432. View publication
- Kazeminasab F, Miraghajani M, Ahmadinejad S, et al. Effects of L-carnitine supplementation on markers of exercise-induced muscle damage in healthy adults: a systematic review and meta-analysis of randomized controlled trials. Adv Exerc Health Sci. 2025;2(2):94-107. View publication
- Pooyandjoo M, Nouhi M, Shab-Bidar S, Djafarian K, Olyaeemanesh A. 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. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product. A varied and balanced diet and a healthy lifestyle are important.
