Erectile dysfunction or "ED" is defined as the inability to obtain or maintain an erection sufficient for satisfactory sexual activity. It is the most widely studied disorder of male sexual function. Other less publicized disorders include: disorders of sexual desire, ejaculatory and orgasm disturbances, as well as disorders involving penile pain or curvature. ED is highly prevalent in the adult male population, and may effect as many as 50% of men between the ages of 40-70 years old.

In order to develop an erection, blood must be able to rapidly enter the male penis through two small arteries that course through the lower pelvis, just under the scrotum. The trigger for this blood flow event lies within the muscles that line the inside of the penis which relax involuntarily when there is sexual stimulation. Over the last 15 years a large amount of research has gone into defining the physiology of male erections. What has become clear is that for a man to develop a satisfactory erection there must be adequate blood flow, a well functioning nervous system, and a reasonable level of circulating male hormones such as testosterone. Diseases that affect any of these body systems can cause ED. In fact, population studies have confirmed that age, high blood pressure, diabetes, heart disease, cigarette smoking, excessive alcohol consumption, and low male hormone levels are significant risk factors for the development of ED. In addition, there are a large number of medications that are used to treat these disorders which may also cause ED.

Prior to the late 1990s, the only treatment available to men with ED were medications that could be injected directly into the penis, a vacuum canister that was applied to the outside of the penis and created an erection through suction, and surgical placement of a prosthetic device into the penis and scrotum. While these treatment options are still available, and provide very satisfactory results in many men with ED, they are not the first choice for the majority of men with ED. What was clearly needed was a pill that could be taken to improve erections.