CoQ10 is categorized as “likely safe” in Natural Medicines Comprehensive Database and has been safely used in studies lasting up to 30 months. CoQ10 is generally well-tolerated. In clinical studies, there have been no reports of significant adverse effects. CoQ10 may cause gastrointestinal side effects such as nausea, vomiting, diarrhea, appetite suppression, heartburn, and epigastric discomfort. Allergic rash has also been reported.
Evidence from well-designed randomized, controlled human clinical trials indicates that the upper level for supplements for CoQ10 is 1200 mg/day.