CoQ10 is well tolerated in most patients. Clinical studies mentioned here show that 100-200 mg per day rarely had any adverse side effects. For example, an Italian study conducted on the safety and efficacy of CoQ10 in the treatment of heart failure, 78% of patients received 100 mg per day and 22% received 50-150 mg per day. Ten adverse reactions were reported by 8 out of 1,113 patients studied (Baggio et al., 1993)
However, a study conducted investigating the effectiveness in the treatment of Huntington's disease where high doses of 600 mg to 1200 mg per day were used, some patients report headache, heartburn, fatigue, and increased involuntary movements (Feigin et al., 1994)
A study conducted with 200 mg 4 times per day for the treatment of Parkinson's disease reported "mild, transient changes in the urine" (Shults et al., 1998).
Coenzyme Q10 may change the insulin requirements of people with diabetes. Talk to your physician if you have diabetes and are planning to take CoQ10.