Potentially serious side effects from Coenzyme Q10 include heart attack, fainting, insomnia and abnormal breathing, states Mayo Clinic. People with diabetes should use CoQ10 cautiously, as it can affect blood sugar levels. CoQ10 can affect heart rate, so people who take related medications should speak to a physician before taking it.
Some people may be allergic to ubiquinone, which is the active ingredient in CoQ10. Signs of an allergic reaction include swelling of the face, mouth, tongue or throat, tightness in the chest, and difficulty breathing. Other signs of an allergic reaction include itching, rash and hives, according to Drugs.com. It is important to use caution when using CoQ10 in high doses for an extended period. Taking more than 300 milligrams of CoQ10 every day can cause changes in liver enzyme levels, states Mayo Clinic.
Other possible side effects of CoQ10 include dizziness, nausea, diarrhea and sore throat. Some people may experience an upset stomach or stomach pain. CoQ10 makes some people feel irritable. Additional possible side effects include sensitivity to light, respiratory tract infection, symptoms similar to the flu and constipation. Other possible side effects include heartburn, gas, head pressure and headache. Runny nose, sinus inflammation, loss of appetite and changes in attention are possible side effects of CoQ10, notes Mayo Clinic.