Viagra: Who What How And Why?

James Pouncey
Dr. Owen
Drugs and Behavior
May 2, 2000
Viagra: What, Who, Why, and How
Sildenafil Citrate, more commonly known by the brand name Viagra, has become one of the most commonly prescribed drugs in America. Viagra is used to treat erectile dysfunction, also known as impotence. Erectile Dysfunction is best defined as the inability to maintain an erection necessary for sexual satisfaction. Sildenafil has been shown to be effective for a “broad range ED patients, including those with a history of mellitus, depression, … hypertension, diabetes, …and spinal chord injury” (Sildenafil Citrate, par. 29).

Pharmacokinetics
Viagra is taken orally. It is quickly absorbed into the system and usually reaches its peak concentration in about 30 to 120 minutes. However, as with most orally administered drugs, taking the pill on a full stomach will slow down its absorption (Sildenafil Citrate, par. 5). Viagra is metabolized in the liver by the cytochrome p450 system. It is converted into a metabolite that has properties similar to that of the pre-metabolized drug. Viagra and its metabolite have a “terminal half-life of about 4 hours” (Sildenafil Citrate, par. 4). Finally, Viagra is excreted primarily through feces in the form of metabolites (Sildenafil Citrate, par. 7).
Mechanism of Action
Sildenafil is a pharmaceutical breakthrough not only because it is the most effective treatment for erectile dysfunction available, but also because it is “the first in a new class of medications” (FDA, par. 3). But to understand how this drug works, it is necessary to explain the physiology of erection in the normally functioning male. Under normal circumstances, sexual excitement results in the release of nitric oxide. Next, the nitric oxide will activate guanylate cyclase, an enzyme. The effect of this enzyme is an increase in the amount of cylic guanosine monophosphate. This increase produces the relaxation of the smooth muscle in the corpus cavernosum, in the penis. This relaxation is necessary to allow the inflow of blood, which is what causes an erection to occur.
However, in a male with erectile dysfunction, phosphodiesterase type 5 is “responsible for degradation of cylic guanosine monophosphate” (FDA, par. 3). Thus, the smooth muscle is unable to relax to allow the inflow of blood. This results in the inability to gain and maintain and erection. Sildenafil works by inhibiting the phosphodiesterase type 5. As a result, there is no inhibition of the cylic guanine monophosphate. Thus the smooth muscle is able to relax and allow the inflow of blood necessary for erection (FDA, par. 4).
Behavioral effects
Thus by examining the mechanisms of action, the myth that Sildenafil causes sexual excitation or that the drug is an aphrodisiac, is dispelled. Rather, it is understood that the drug simply allows the natural response to sexual excitation to occur, rather than causing the excitation itself. Because of the drug’s effects, the use of the drug is associated with some behavioral changes. Those who suffer from erectile dysfunction often have anxiety and lack on confidence in regards to sexual functioning. They may in fact avoid situation in which they may feel sexually inadequate. Obviously, this can result in a less than satisfactory sex-life. Because Sildenafil is able to provide normal sexual functioning for most of those who take it, it naturally follows that users can experience a renewal in their self-confidence and a decrease of their sexual anxieties regarding problems with erection, as they are able to maintain and enjoy a normal sex-life. [Note: none of the material in the preceding paragraph was taken from another source.]
Physiological Side Effects
Like most drugs, Sildenafil does have side effects, but the side effects “when they occur, are usually mild and temporary” (FDA, par. 6). One of these side effects is a temporary change in blue/green color perception. This change in perception occurs because Sildenafil inhibits PDE6, “which is involved in phototransduction in the retina” (Sildenafil Citrate, par. 17). Also, Sildenafil causes a decrease in blood pressure, though the cause of this is unknown (Sildenafil Citrate, par.13); further common side effects include “headache, flushing, and upset stomach” (FDA, par. 6). Finally, it is recommended that those who take nitrates (such as nitroglycerin, often used for the treatment of angina) not take Sildenafil as it was shown to “potentiate [increase] the