How do drugs like Viagra work?

Viagra (sildenafil citrate), a prescription medication for the treatment of erectile dysfunction, is the first pill available that's been proven to improve erections in most men with impotence.  
 
Since its introduction in March 1998, no other therapy for ED has achieved such wide public recognition. Viagra doesn't improve erections in normal men, only in those with difficulty in achieving or maintaining erections sufficient for sexual intercourse due to a true medical problem. It is not an aphrodisiac (sexual stimulant) and will not increase sexual desire. Unlike other treatments for erectile dysfunction, Viagra requires sexual stimulation to function. Without this stimulation, Viagra won't have any effect. 
 
Viagra works by blocking an enzyme found mainly in the penis that breaks down chemicals produced during sexual stimulation that normally produce erections. Viagra allows these chemicals of arousal to survive longer and improves erectile function. That is also why sexual stimulation is necessary for Viagra to work. 
 
In general, Viagra works successfully in about 65-70% of all impotent men.
 
Viagra works best if taken about 30-60 minutes before sexual activity. Only 1 tablet should be taken per day. It should be taken on an empty stomach. Increasing the dosage of Viagra beyond the recommended amounts will not improve the response and will only result in greater side effects. 
 
Several drugs very similar to Viagra have recently been approved by the FDA. These drugs, called vardenafil (Levitra) and tadalafil (Cialis), have essentially the same activity and general precautions as Viagra. Levitra may be taken with food where Viagra needs to be taken on an empty stomach. Cialis has a much longer duration of improved erection function (up to 24-36 hours) compared with Viagra and Levitra (up to 4-6 hours).