Coenzyme Q10 (also called CoQ10) is a fat-soluble substance, similar to certain vitamins, that is found in the human body. CoQ10 is necessary for the basic function of cells in the body. It is involved in energy production and exhibits antioxidant activity. However, levels of CoQ10 tend to decrease over time. This decrease has been linked to the onset of several diseases, including heart disease, Parkinson's disease (PD), and cancer. The effect of supplementing CoQ10 on certain diseases has been studied. High doses (1200mg per day) slowed the rate of motor dysfunction in 80 individuals with early PD over a 16 month period. No differences were seen at doses of 300mg and 600mg per day. CoQ10 also improved those persons' ability to perform their normal daily activities. This study shows that higher doses of CoQ10 than are typically used may be needed for maximum effectiveness at slowing the progression of PD. The usual dosage range of CoQ10 is 30mg to 300mg daily. While side effects are uncommon, abdominal pain and nausea may occur.