L-carnitine side effects

L-carnitine is generally well tolerated. The side effects most often reported are gastrointestinal — nausea, stomach cramping, diarrhea — along with restlessness, and a fishy odor to the breath, sweat or urine in some people. None of these is dangerous on its own, but they're worth knowing before you take a dose bigger than your stomach is used to.

The side effects reported in research

The Drugs and Lactation Database (LactMed), maintained by the National Institutes of Health, summarizes levocarnitine's tolerability this way: it's "generally well tolerated in adults with occasional gastrointestinal upset and restlessness," and "a fishy odor to the breath, sweat and urine has been reported." Neither effect is described as dose-specific in that summary, but a mechanism helps explain the gastrointestinal symptoms: a 2004 pharmacokinetics review found that supplemental L-carnitine is absorbed far less efficiently than dietary carnitine, and the portion that isn't absorbed is fermented by bacteria in the large intestine — the same general mechanism behind gas and loose stools with other poorly absorbed supplements.

Liquid and flavored formats

Some liquid or flavored L-carnitine products are sweetened with sugar alcohols (like sorbitol) rather than sugar, to keep the calorie count down. Sugar alcohols are independently known to cause gas, bloating and loose stools in some people at higher intakes — that's a property of the sweetener, not the carnitine, but it's easy to blame the wrong ingredient if you switch from a capsule to a flavored liquid and notice new stomach symptoms.

What isn't established

Restlessness is the specific effect noted in the LactMed summary above — not sleepiness. No study identified for this article specifically tested or reported drowsiness, and none specifically measured irritability as an outcome, so there's no research-based answer either way for either of those.

Questions people ask

What are the side effects of L-carnitine?

The most commonly reported are gastrointestinal upset (nausea, cramping, diarrhea) and restlessness, along with a fishy body odor in some people, according to the NIH's LactMed database.

Does L-carnitine make you sleepy?

No. The documented effect in the research summarized above is restlessness, not sedation, and there's no research finding a sleep-inducing effect at any dose.

Can L-carnitine cause irritability?

No study identified for this article measured irritability specifically. Restlessness is the closest documented effect, and it's reported only occasionally.

Does L-carnitine 3000 (a higher dose) have more side effects?

A higher milligram number on a label means more of the dose goes unabsorbed and reaches the large intestine, which is the mechanism behind the gastrointestinal symptoms described above — so a larger dose is a plausible way to increase the chance of stomach upset, even though the research summarized here doesn't report side-effect rates broken out by exact dose.

When to check with a doctor

If gastrointestinal symptoms are severe, persistent, or come with other symptoms, stop and talk to a doctor rather than assuming it's L-carnitine. If you're pregnant, nursing, or take prescription medication, talk to a doctor before starting any supplement — see who should not take L-carnitine? and L-carnitine and medications: what to check.

Related reading: L-carnitine dosage and what is L-carnitine?.