http://www.mayoclinic.org/drugs-supplements/coenzyme-q10/safety/hrb-20059019
CoQ10 is likely safe when up to 3,000 milligrams is taken by mouth daily for up to eight months in healthy people. Nanoparticular CoQ10 is safe at doses of 300 milligrams daily for up to three months. CoQ10 is safe when taken by mouth daily in recommended doses in people who are about to have heart surgery, or those who have heart disease, gum disease, muscle wasting/weakness, Parkinson's disease, Alzheimer's disease, cancer, diabetes, or HIV/AIDS. CoQ10 is also likely safe in healthy people when taken for exercise performance and in children who have chemotherapy side effects. Idebenone is likely safe in doses of up to 60 milligrams per kilogram daily.