No serious toxicity associated with the use of coenzyme Q10 has been reported. Reviewed in [1-4] 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.
Certain lipid -lowering drugs, such as the statins (lovastatin, pravastatin, and simvastatin) and gemfibrozil, as well as oral agents that lower blood sugar, such as glyburide and tolazamide, cause a decrease in serum levels of coenzyme Q10 and reduce the effects of coenzyme Q10 supplementation.[7,8] Reviewed in [1,9] Beta-blockers (drugs that slow the heart rate and lower blood pressure) can inhibit coenzyme Q10-dependent enzyme reactions. Reviewed in  The contractile force of the heart in patients with high blood pressure can be increased by coenzyme Q10 administration. Reviewed in  Coenzyme Q10 can reduce the body’s response to the anticoagulant drug warfarin. Reviewed in  Finally, coenzyme Q10 can decrease insulin requirements in individuals with diabetes. Reviewed in References
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