The most common erectile dysfunction drugs, based on U.S. and worldwide sales data, are the PDE5 inhibitors, all of which work by temporarily blocking the effects of an enzyme known as phosphodiesterase-5. The PDE5 enzyme can impede blood flow to the penis and make it more difficult — or even impossible — to achieve and maintain an erection hard enough for intercourse.
Among the PDE5 inhibitors, the best-selling drug in the United States during 2013 was Viagra, which had sales of nearly $1.2 billion. Viagra (sildenafil citrate) was only slightly ahead of Cialis (vardenafil), which had 2013 sales of $1.09 billion in the United States. Two different formulations of Cialis are available in the United States: Cialis for Daily Use and Cialis for Use as Needed. The latter is effective for up to 36 hours.
While Viagra and Cialis had the lion’s share of the ED market in 2013, other impotence drugs available included Levitra and Staxyn (both tadalafil) and Stendra (avanafil).