Safety · Ingredient

L-carnitine side effects and safety

Short answer

L-carnitine is generally well tolerated up to about 2 g a day, the observed safe level for long-term use. Around 3 g a day it can cause nausea, vomiting, stomach cramps, diarrhoea and a fishy body odour. People with thyroid problems, seizure disorders or kidney disease, or on blood thinners, should ask a doctor first.

Updated 14 November 2026By Get Waisted editorial4 sources · 3 min

Common side effects (and at what dose)

L-carnitine side effects by dose (NIH Office of Dietary Supplements; Hathcock and Shao, 2006)
Daily amount	What's reported
Up to 2 g	Observed safe level for long-term use; side effects uncommon
About 3 g	Nausea, vomiting, stomach cramps, diarrhoea, fishy body odour
Any dose, specific groups	Muscle weakness in people with uraemia (kidney failure); seizures in people with seizure disorders

The fishy smell comes from trimethylamine (TMA), which gut bacteria make from L-carnitine that isn't absorbed. It goes away when you lower the dose.

Who should avoid L-carnitine or ask a doctor first?

Pregnant or breastfeeding: not enough safety data on supplements.
Hypothyroidism or on thyroid medicine: L-carnitine may blunt thyroid hormone's action in cells.
Seizure disorder: seizures have been reported with L-carnitine in people with a history of them.
Kidney disease: carnitine builds up when the kidneys can't clear it; in kidney failure it's prescribed only under supervision.
On blood thinners (warfarin, acenocoumarol): case reports of a higher INR, meaning more bleeding risk.
On valproic acid or other anti-seizure drugs: these lower carnitine levels, so dosing is a doctor's call.

Is L-carnitine bad for the liver, kidneys or hair?

Liver

We found no good evidence that normal doses harm a healthy liver. L-carnitine is even being studied in fatty liver disease, with mixed results. If you have liver disease, ask your doctor.

Kidneys

Healthy kidneys clear extra carnitine in urine. In kidney disease they can't, so L-carnitine should only be taken under medical supervision.

Hair loss

There's no clinical evidence that L-carnitine causes hair loss. The worry comes from the idea that it raises DHT, which isn't established in humans. Lab and small human studies have even tested it on the scalp as a hair-growth treatment.

L-carnitine and TMAO: the heart question

A 2013 study in Nature Medicine found that gut bacteria turn L-carnitine into TMA, which the liver converts to TMAO, and that TMAO was linked to clogged arteries in mice and to heart risk in people. Meat eaters made much more TMAO from L-carnitine than vegans did (Koeth et al., 2013).

Whether supplement-level L-carnitine raises heart risk in healthy people is still an open question. The NIH says the implications "are not well understood". It's a reason to stay at moderate doses and not to stack several carnitine products.

Is it safe to take L-carnitine long term?

Trials lasting several months at 2 g a day or less have not found serious problems. A safety review concluded the evidence is strong up to 2 g a day for chronic use, and that data above that are not enough for a confident long-term verdict (Hathcock and Shao, 2006).

Red flags on any L-carnitine product
Red flag	Why
No amount per serving on the label	You can't know if you're near 2 g or far above it
D- or DL-carnitine	The D form is inactive and can block L-carnitine
"Burns belly fat" or "lose 5 kg"	Trials show no waist effect and about 1.2 kg on average
Stacked stimulants	Many "fat burners" add large doses of caffeine or herbs
No test report	Nothing shows what's actually in it

This page is general information, not medical advice. Talk to a doctor before starting a supplement or a weight-loss plan, especially if you're pregnant, on medication or managing a health condition.

Questions people ask

What are the side effects of L-carnitine?

At about 3 g a day: nausea, vomiting, stomach cramps, diarrhoea and a fishy body odour. It may also cause muscle weakness in people with kidney failure and seizures in people with seizure disorders.

Is L-carnitine safe?

For healthy adults, generally yes at up to 2 g a day, the observed safe level for long-term use. Ask a doctor first if you're pregnant, on medication, or have a thyroid, kidney or seizure condition.

Does L-carnitine cause hair loss?

There's no clinical evidence that it does. The claim rests on an unproven link to DHT.

Is L-carnitine bad for the kidneys?

Not for healthy kidneys, which clear the excess. People with kidney disease should take it only under medical supervision.

Can L-carnitine cause diarrhoea?

Yes, mostly at around 3 g a day or when a large dose is taken at once. Lowering or splitting the dose usually stops it.

Is L-carnitine safe for women?

Yes for most healthy women at normal doses. Pregnant or breastfeeding women, and anyone with a thyroid condition, should ask a doctor first.

Does L-carnitine increase heart rate?

It isn't a stimulant and doesn't usually raise heart rate. If a "fat burner" makes your heart race, check it for caffeine or other stimulants.

Sources

01NIH Office of Dietary Supplements: Carnitine, fact sheet for health professionals.Government (US)
02Hathcock J.N., Shao A. (2006). Risk assessment for carnitine. Regulatory Toxicology and Pharmacology 46(1):23–28.Peer-reviewed · safety
03Koeth R.A. et al. (2013). Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nature Medicine 19:576–585.Peer-reviewed · mice and humans
04Talenezhad et al. (2020). Effects of L-carnitine supplementation on weight loss and body composition: a systematic review and dose-response meta-analysis of 37 randomized controlled clinical trials. Clinical Nutrition ESPEN 37:9–23.Peer-reviewed · meta-analysis · humans

L-Carnitine Water · first batch

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Every batch of Get Waisted L-Carnitine Water gets a lab report: the L-carnitine measured, and what isn't in it. Reserve a pack free.