Guide
Does L-carnitine cause diarrhea, bloating or nausea?
L-carnitine's most commonly reported side effect is gastrointestinal upset, including diarrhea in some people, likely because most of a supplemental dose isn't absorbed and is instead fermented by bacteria in the large intestine.
Yes, for some people. Gastrointestinal upset — which can include diarrhea, loose stools or an upset stomach — is the most commonly reported L-carnitine side effect. The likely reason: most of a supplemental dose isn't absorbed and instead reaches the large intestine, where gut bacteria ferment it.
Why it happens
A 2004 pharmacokinetics review in the Annals of the New York Academy of Sciences found that supplemental L-carnitine doses of 0.5 to 6 g are absorbed at only 14-18%, far below the 54-87% absorption rate of the small amounts of carnitine naturally in food. The unabsorbed majority of a supplement dose isn't simply excreted — it's fermented by microorganisms in the large intestine. That fermentation process is a plausible, well-established mechanism for gas, bloating and loose stools, and it's consistent with why gastrointestinal symptoms tend to show up more at higher doses: a bigger dose means a bigger unabsorbed fraction reaching the colon.
Separately, the NIH's LactMed (Drugs and Lactation Database) monograph on levocarnitine describes it as generally well tolerated in adults, with gastrointestinal upset and restlessness as the main effects reported, alongside a fishy odor to the breath, sweat or urine in some people. See L-carnitine side effects for the full list.
Does taking it with food change anything?
We haven't found a trial that directly tested L-carnitine on an empty stomach versus with food for gastrointestinal tolerance. The fermentation mechanism above would apply either way, since it happens after the small intestine's absorption window has closed, not because of what else is in your stomach at the time. If stomach upset shows up for you, splitting a dose or taking it with a meal is a reasonable thing to try, even though no specific trial has measured whether it helps.
Questions people ask
Why does L-carnitine make you poop?
The leading explanation in the research is that most of a supplemental dose isn't absorbed in the small intestine and instead reaches the colon, where bacterial fermentation of the unabsorbed carnitine can trigger loose stools or diarrhea.
Can L-carnitine cause constipation instead?
No research identified for this article measured constipation as an outcome. The documented gastrointestinal effect runs the other direction — loose stools, not constipation.
Can L-carnitine cause stomach problems in general?
Gastrointestinal upset is the specific, documented effect. "Stomach problems" more broadly isn't a term the research uses, but nausea and general GI discomfort are covered under the same reporting.
Does L-carnitine cause heart problems?
No research identified for this article found L-carnitine supplementation causes heart problems in healthy adults at the doses studied. If you have an existing heart condition or take heart medication, ask your doctor before adding any supplement — see L-carnitine and medications: what to check.
Can L-carnitine cause vertigo?
No research identified for this article reported vertigo as a side effect. If you experience dizziness after starting any supplement, stop and check with a doctor.
Related reading: L-carnitine side effects, L-carnitine dosage, and is it safe to take L-carnitine every day?.
Sources
- Levocarnitine L-carnitine is generally well tolerated, with occasional stomach upset, restlessness, and a reported fishy body odor in some people.
- Kinetics, pharmacokinetics, and regulation of L-carnitine and acetyl-L-carnitine metabolism Carnitine from food is absorbed far more efficiently than carnitine from a supplement, and acetyl-L-carnitine behaves somewhat differently in the body than plain L-carnitine.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.