Possible side effects of coenzyme Q10 include upper abdominal pain, dizziness, nausea and fatigue. Some people may experience heartburn, irritability, rashes and sensitivity to light. Taking 100 milligrams or more may cause mild insomnia, and taking 300 milligrams or more long-term may raise liver enzyme levels, states WebMD.
Additional possible side effects of coenzyme Q10 include a loss of appetite, diarrhea or an upset stomach. Some people may have a severe allergic reaction to ubiquinone, which is the active ingredient in coenzyme Q10. Signs of an allergic reaction include hives, rash, itching and difficulty breathing. Swelling of the face, tongue, mouth or lips are also signs of an allergic reaction. If an allergic reaction occurs, a physician should be contacted immediately, notes Drugs.com.
People with diabetes should speak to a doctor before taking coenzyme Q10, as it may decrease the need for insulin. The long-term side effects of most dietary supplements are unknown, as of 2015. Because dietary supplements are not regulated like traditional medicines, it is important to use only reliable products as recommended by a health care professional. People who plan on taking coenzyme Q10 should tell a doctor or physician about any other medications or dietary supplements being taken, as coenzyme Q10 may interact with some of them, according to WebMD.