Sildenafil ('Viagra') is a selective PDE5 inhibitor (see the Neurophysiology section). During sexual stimulation, nerves in the penis release nitric oxide (NO). This molecule in turn causes an increase in the amounts of cGMP in the corpora cavernosa. The elevation of cGMP causes vasodilation allowing increased blood volume to flow to the penis causing an erection. Sildenafil works by inhibiting the enzyme that breaks the cGMP down.
Sildenafil is supplied in 25, 50 and 100mg doses of sildenafil citrate. The tablet is taken one hour before sexual activity. It does not cause an erection unless the man is sexually stimulated.
The most common adverse side effects of this treatment are:
There are other oral medications available such as: