A class of erectile dysfunction drugs that includes Viagra has been linked to a slight increase in the risk of the skin cancer melanoma, according to a study Tuesday. While the research in the Journal of the American Medical Association stops short of proving that the drugs cause melanoma, researchers said their findings warrant a closer look at medications known as PDE5 (phosphodiesterase type 5) inhibitors. Of those patients, 11 percent had filled at least one prescription for a PDE5 inhibitor to treat erectile dysfunction.