Carnitine is well tolerated across 991 trials, and the usual side effects are minor and digestive. There are also 539 papers on something that is not a side effect at all.
It is a metabolic pathway, and 2 landmark studies made it famous.
Quick Answer: What Are L-Carnitine's Side Effects?
In trials the reported effects are mild: nausea, stomach upset, loose stools, and a fishy body odour at higher doses. The bigger question is not a side effect but a pathway. Gut bacteria convert carnitine into trimethylamine, which the liver turns into TMAO. A 2013 Nature Medicine paper showed TMAO accelerated atherosclerosis in mice, and that omnivores produce far more of it than vegans do.
Key Takeaways
- The 3 reported side effects are nausea, cramps and loose stools.
- 1 further effect, a fishy body odour, appears at higher doses.
- 539 PubMed papers now cover the trimethylamine and TMAO pathway.
- In mice TMAO accelerated atherosclerosis, and that 1 result is not human.
- In people, omnivores produce far more TMAO than the 1 vegan group did.
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The Ordinary Side Effects
Across 991 trials the tolerability picture is undramatic, which is worth saying before the more interesting part.
- Most common: Nausea, stomach cramps or loose stools.
- Dose-related: A fishy body odour, from trimethylamine.
- Occasional: Restlessness if taken late in the day.
- Rare: Seizure reports, which is why it is a label caution.
None of that is the reason this article exists. What the compound is doing is in our carnitine overview.
The TMAO Pathway
This is the part that turns a simple supplement question into a genuinely open one.
| Step | What happens | Where shown |
|---|---|---|
| 1. You swallow carnitine | Some reaches the colon undigested | Human |
| 2. Gut bacteria act on it | It becomes trimethylamine, or TMA | Human |
| 3. The liver oxidises TMA | It becomes TMAO | Human |
| 4. TMAO accumulates | Atherosclerosis accelerated | Mice |
The 2013 Nature Medicine paper established the chain and is careful about which species showed what.[1]Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis — Nature Medicine (2013) View source The atherosclerosis was in mice. The step measured in people was that omnivorous subjects produced considerably more TMAO than vegans and vegetarians.
What the Human Follow-Up Added
A 2019 study went looking for the missing link in people rather than in mice.
- The intermediate: Gamma-butyrobetaine, an atherogenic step.
- Who was compared: Omnivores against vegans.
- What differed: The gut microbial pathway, not the dose swallowed.
- The implication: Your diet history matters as much as the capsule.
The 2019 work confirmed the pathway operates in humans and identified where the intermediate fits.[2]l-Carnitine in omnivorous diets induces an atherogenic gut microbial pathway in humans — Journal of Clinical Investigation (2019) View source What that means for taking it to lose weight is in the weight-loss evidence.
How Worried to Be
Being accurate here matters more than being alarming or reassuring, so this is stated as a balance rather than a verdict. Amounts and formats are in amounts and formats.
- Established: The pathway is real and operates in people.
- Not established: That supplement carnitine causes events in humans.
- Context: Red meat supplies carnitine daily to most omnivores.
- Where risk concentrates: People already at cardiovascular risk.
Nobody has run a trial giving carnitine to people for years and counting heart attacks. Until somebody does, the pathway is a reason for caution rather than a proven harm.
The weight literature, by contrast, has been pooled 4 times, and even there the reviewers describe the clinical findings as contradictory.[3]Beneficial effects of l-carnitine supplementation for weight management in overweight and obese adults: An updated systematic review and dose-response meta-analysis — Pharmacological Research (2021) View source
Who Should Be Careful
Some of these come from the label and some from the pathway above.
- Kidney disease or dialysis: Carnitine handling changes entirely.
- On warfarin: Carnitine has been reported to affect its action.
- Seizure disorders: A standard caution on the 2 common forms.
- Underactive thyroid: Carnitine may blunt thyroid hormone action.
For everyone outside those groups the capsule form has an undramatic tolerability record.
Speak to a Clinician Before Starting Carnitine If
- You have kidney disease or are on dialysis, where carnitine handling changes
- You take warfarin, because carnitine has been reported to affect its action
- You have a seizure disorder, which is 1 of the standard label cautions
- You have an underactive thyroid, since carnitine may blunt thyroid hormone action
- You are pregnant or breastfeeding, where the trials give no guidance
Frequently Asked Questions
What are the side effects of L-carnitine? +
Mild and mostly digestive, with 3 common ones: nausea, cramps and loose stools. A fishy body odour appears in some people at higher doses.
Why does L-carnitine cause a fishy smell? +
Because gut bacteria turn some of it into trimethylamine. That is step 1 of the TMAO pathway.
Is L-carnitine bad for your heart? +
Not proven, and not dismissed. 539 papers cover the TMAO pathway, and the atherosclerosis result was in mice rather than people.
What is TMAO and should I worry? +
A compound formed from carnitine by gut bacteria and the liver, across 539 papers. It is a reason for caution, not a demonstrated harm in humans.
Do vegans make less TMAO? +
Yes. That was 1 of the human findings in the 2013 study, and the 2019 follow-up confirmed the pathway differs by diet.
Can L-carnitine cause seizures? +
Seizure reports are why it carries that caution. It is rare, and it is 1 reason to ask if you have a seizure disorder.
Is carnitine safe with kidney disease? +
Ask the clinician managing it. Kidney handling of carnitine changes, which makes this 1 of the clearest exclusions.
Does carnitine interact with warfarin? +
It has been reported to affect warfarin's action, so the 2 should not be combined without your prescriber.
Should I stop eating red meat if I take it? +
That is a bigger dietary question than 1 capsule. Red meat is the main food source of the same compound.
Is the liquid gentler on the stomach? +
There is no trial comparing the 2 for tolerability. Taking either with food is the usual way to reduce upset.
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