Some studies report a “fishy” odor emanating from breath and sweat following supplementation with L-carnitine, which is likely due to the formation of trimethylamine.[1] Dry mouth and gastrointestinal discomfort have also been reported in some studies using L-carnitine,[2] while gastrointestinal problems (including nausea and gastric pain) are a common adverse effect of propionyl-L-carnitine.[3][4] However, several meta-analyses conclude that L-carnitine and its derivatives are generally safe and well‐tolerated.[5][3][4] Furthermore, supplementation of up to 2000 milligrams (mg) per day is considered safe for human consumption.[6] Some human studies have even used doses of L-carnitine as high as 6000 mg/day without serious side effects,[7] but a thorough safety evaluation of such high doses is lacking.[6]
What are L-carnitine’s main drawbacks?
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•Last Updated: May 16, 2025
Supplementing with L-carnitine may cause a "fishy" odor in the breath and sweat, dry mouth, and gastrointestinal discomfort; nausea and gastric distress are the common adverse effects of propionyl-L-carnitine. Despite these drawbacks, meta-analyses suggest that L-carnitine and its derivatives are generally safe and well-tolerated, and daily doses up to 2,000 mg are considered safe.