Coenzyme Q10 (PDQ®): Complementary and alternative medicine - Health Professional Information [NCI] - Adverse Effects
No serious toxicity associated with the use of coenzyme Q10 has been reported. Reviewed in [1,2,3,4]Doses of 100 mg /day or higher have caused mild insomnia in some individuals. Reviewed in Liver enzyme elevation has been detected in patients taking doses of 300 mg/day for extended periods of time, but no liver toxicity has been reported. Reviewed in  Researchers in one cardiovascular study reported that coenzyme Q10 caused rashes, nausea, and epigastric (upper abdominal) pain that required withdrawal of a small number of patients from the study. Other reported side effects have included dizziness, photophobia (abnormal visual sensitivity to light), irritability, headache, heartburn, and fatigue.
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