Side effects of CoQ10 are generally mild and include loss of appetite, stomach upset, diarrhea, nausea and vomiting, according to WebMD. The substance may also cause an allergic skin rash in some individuals, and it may lower blood pressure.
High doses of more than 300 milligrams daily may affect levels of enzymes in the liver, explains Mayo Clinic. CoQ10 may also increase the risk of bleeding, decrease mobility in patients with mitochondrial disorders, cause headaches and affect blood sugar levels. Other potential side effects include bronchitis, abnormal breathing, flu-like symptoms, insomnia and heartburn, as well as irritability, light sensitivity, hearing loss and head pressure.
Most adults tolerate CoQ10 when taking the supplement orally or applying it to the gums, and if side effects do occur, patients can generally reduce them by dividing the daily dose into two or three smaller doses, states WebMD. Children should only take CoQ10 under medical supervision, and pregnant women should avoid the supplement until 20 weeks to delivery, at which time they may take it safely twice daily. Patients undergoing chemotherapy or taking blood pressure-lowering medications should consult a doctor before taking CoQ10 as the substance may interfere with chemotherapy drugs and lower blood pressure.