Heart failure is a term used to describe the state that develops when the heart cannot maintain adequate cardiac output or can do so only at the expense of overfilling the heart chambers. People with heart failure commonly experience a relapsing and remitting disease course, with periods of stability and episodes of decompensation (failure to cope with heart damage) leading to worsening symptoms that necessitate hospitalisation. Treatment options for heart failure range from drugs to heart transplantation, with each having its own limitations. Coenzyme Q10 (or ubiquinone) has been suggested as a treatment option in some trials. Coenzyme Q10 is a non-prescription nutritional supplement. It is a fat-soluble molecule that has a role in energy production within the cells of the body. It may also have antioxidant properties. Low levels of conequme Q10 may be related to the severity of heart failure. Coenzyme Q10 has been found in all tissues and organs in the body, with the highest concentrations in the heart. Emerging data have suggested that the harmful effects of reactive oxygen species are increased in patients with heart failure and coenzyme Q10, given its antioxidant activity, may help to reduce these toxic effects, which damage the components of the cardiac cells and disrupt cellular signalling. Coenzyme Q10 also has a role in stabilising myocardial calcium-dependent ion channels and preventing the consumption of metabolites essential for adenosine-5'-triphosphate (ATP) synthesis. The concentration of coenzyme Q10 has been inversely related to the severity of heart failure. Supplementation with coenzyme Q10 may improve heart failure. Coenzyme Q10 is sometimes used because it is thought to have an acceptable safety profile with no significant side effects.