No serious side effects have been reported from the use of coenzyme Q₁₀. The most common side effects include the following:

• Insomnia (being unable to fall sleep or stay asleep).
• Higher than normal levels of liver enzymes.
• Rashes.
• Nausea.
• Pain in the upper part of the abdomen.
• Dizziness.
• Feeling sensitive to light.
• Feeling irritable.
• Headache.
• Heartburn.
• Feeling very tired.

It is important to check with health care providers to find out if coenzyme Q₁₀ can be safely used along with other drugs. Certain drugs, such as those that are used to lower cholesterol, blood pressure, or blood sugar levels, may decrease the effects of coenzyme Q₁₀. Coenzyme Q₁₀ may change way the body uses warfarin (a drug that prevents the blood from clotting) and insulin.

As noted in Question 1, the body uses coenzyme Q₁₀ as an antioxidant. Antioxidants protect cells from free radicals. Some conventional cancer therapies, such as anticancer drugs and radiation treatment, kill cancer cells in part by causing free radicals to form. Researchers are studying whether using coenzyme Q₁₀ along with conventional therapies has any effect, good or bad, on the way these conventional therapies work in the body.

Coenzyme Q₁₀ is sold as a dietary supplement and is not approved by the FDA for use as a cancer treatment. In the United States, dietary supplements are regulated as foods, not drugs. This means that approval by the FDA is not required before coenzyme Q₁₀ is sold, unless specific health claims are made about the supplement. Also, the way companies make coenzyme Q₁₀ is not regulated. Different batches and brands of coenzyme Q₁₀ supplements may be different from each other.