Severe allergic side effects associated with oral CoQ10 use include difficulty breathing, swelling of the face and tongue, tightness in the chest and itching, according to Drugs.com. Less severe CoQ10 side effects include insomnia, nausea, dizziness, sensitivity to light and headaches, reports WebMD.
Oral supplementation of up to 3,000 milligrams of CoQ10 daily for eight months or less is generally safe for healthy people. Recommended daily doses of oral CoQ10 are safe for people with heart disease, Parkinson's disease, cancer, diabetes and other chronic conditions, according to Mayo Clinic. CoQ10 may increase the risk of bleeding disorders, especially in persons taking warfarin and other blood thinners, says WebMD.