Evidence explainerCarnitine (L-carnitine, L-carnitine L-tartrate, acetyl-L-carnitine)

L-Carnitine Side Effects: Stomach Upset and Fishy Odor at 3 g, Plus the TMAO Question

Carnitine has no official upper limit, but higher doses have known side effects, two groups need extra care, and gut bacteria turn the unabsorbed part into a compound researchers are still debating.

By Kymata Health editorial teamPublished October 9, 2026Last reviewed October 9, 2026

Medical review pending. Written by our editorial team from the studies cited below; a credentialed clinician has not reviewed it yet. Educational only, not medical advice.

Key takeaways

  • Carnitine has no established tolerable upper intake level [1].
  • Around 3 g/day, carnitine supplements can cause nausea, vomiting, abdominal cramps, diarrhea and a fishy body odor [1].
  • ODS reports carnitine can cause muscle weakness in people with uremia and seizures in people with seizure disorders [1].
  • Only about 14% to 18% of supplemental L-carnitine is absorbed; gut bacteria can turn the rest into TMAO, whose implications are not well understood [1].
  • In a 2003 Cochrane review of acetyl-L-carnitine trials, adverse events did not differ significantly from placebo [2].

The evidence, graded

Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.

  • Grade BWorsenedBody weight, BMI and fat massCarnitine (L-carnitine, L-carnitine L-tartrate, acetyl-L-carnitine) · Meta-analysis · n=2,292 · 2020 · PMID 32359762L-carnitine reduced body weight by about 1.2 kg, BMI by 0.24 kg/m² and fat mass by about 2.1 kg versus control. Waist circumference and body-fat percentage did not change. High-quality trials alone confirmed the weight effect; authors note it especially in adults with overweight or obesity.
  • Grade DImprovedMuscle carnitine and high-intensity exercise performanceCarnitine (L-carnitine, L-carnitine L-tartrate, acetyl-L-carnitine) · RCT · n=14 · 2011 · PMID 21224234Muscle carnitine rose 21% with carnitine and was unchanged in controls; muscle glycogen use at low intensity was 55% lower, and work output in the performance trial rose 11% from baseline with no change in controls.

The common side effects

Carnitine does not have an established tolerable upper intake level [1]. ODS reports that doses of approximately 3 g/day can cause nausea, vomiting, abdominal cramps, diarrhea and a fishy body odor [1].

Trials have used a wide range of amounts: eight of the nine weight-loss trials ODS describes gave 1.8 to 4 g/day of L-carnitine for 30 to 360 days [1].

Absorption and TMAO

Absorption of supplemental L-carnitine is about 14% to 18%, much less than carnitine from food [1]. Some research indicates intestinal bacteria turn unabsorbed carnitine into TMAO and gamma-butyrobetaine, which might increase cardiovascular disease risk [1].

That effect appears more pronounced in people who eat meat than in vegans or vegetarians, and ODS says the implications are not well understood and need more research [1].

Who needs extra care

Beyond the dose-related effects, ODS lists two groups and two kinds of medicine to know about [1]:

  • People with uremia: carnitine can cause muscle weakness [1].
  • People with seizure disorders: carnitine can cause seizures [1].
  • Anticonvulsants (valproic acid, phenobarbital, phenytoin, carbamazepine) reduce blood carnitine levels [1].
  • Long-term pivalate-conjugated antibiotics such as pivampicillin can deplete carnitine, although no cases of illness from the resulting low carnitine have been described [1].

Do you need it at all?

Healthy children and adults do not need carnitine from food or supplements because the liver and kidneys make enough [1]. The body needs about 15 mg/day; a typical omnivorous diet supplies about 24 to 145 mg/day and a vegan diet about 1.2 mg/day [1].

Anyone taking prescription medicines or living with a chronic condition should talk to a pharmacist or clinician before adding carnitine [1]. For the weight-loss evidence, read L-carnitine for weight loss.

Quick answers

Why can L-carnitine cause a fishy smell?

ODS lists a fishy body odor among the effects of doses around 3 g/day [1]. Our sources do not explain the mechanism.

Is L-carnitine bad for your heart?

ODS notes research suggesting gut bacteria convert unabsorbed carnitine into TMAO, which might raise cardiovascular disease risk, and says the implications are not well understood [1].

Is acetyl-L-carnitine easier to tolerate?

The 2003 Cochrane review of 11 acetyl-L-carnitine trials, all in people with Alzheimer's disease, reported various adverse events but no statistically significant difference from placebo [2]. We found no head-to-head tolerance comparison of carnitine forms.

Is L-carnitine safe long term?

No upper limit has been set [1]. The weight-loss trials ODS describes lasted up to 360 days [1], so data beyond about a year are limited.

Keep reading

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References

  1. [1]Carnitine: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2023.
  2. [2]Acetyl-L-carnitine for dementia. Cochrane Database Syst Rev, 2003. PMID 12804452

Carnitine (L-carnitine, L-carnitine L-tartrate, acetyl-L-carnitine) and other dietary supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA. Talk to your clinician or pharmacist before starting a supplement, especially if you take prescription medicines. This page has no product or affiliate links.

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