Sildenafil inhibits an enzyme called phosphodiesterase-5 (PDE5) which destroys
the cGMP. Thus, sildenafil prevents the destruction of cGMP and allows cGMP to accumulate and persist longer. This engorgement occurs when the blood vessels delivering
blood to the penis increase the delivery of blood and the blood vessels carrying blood away from the penis decrease the removal of blood. Under normal conditions, sexual
stimulation leads to the production and release of nitric oxide in the penis. Nitric oxide then activates the enzyme, guanylate cyclase, which causes the production of cyclic
guanosine monophosphate (cGMP). It is the cGMP that is primarily responsible for the erection by affecting the amount of blood that the blood vessels deliver and remove
from the penis. The longer cGMP persists, the more prolonged the engorgement of the penis. It also reduces the pressure in the pulmonary artery in a serious condition
called pulmonary arterial hypertension.

Sildenafil is rapidly absorbed. Maximum observed plasma concentrations are reached within 30 to 120 minutes (median
60 minutes) of oral dosing in the fasted state. When sildenafil is taken with a high fat meal, the rate of absorption is reduced, with an average delay in the time to maximal
concentration of 1 hour. Store this medication at room temperature, 15- 30°C (59-86°F).

Most common side effects include facial flushing (1 in 10), headaches (1 in 6),
stomach pain, nasal congestion, nausea, diarrhea, and an inability to differentiate between the colors green and blue. If any of these side effects persist or worsens, inform
your doctor.

Cimetidine (Tagamet), erythromycin, ketoconazole (Nizoral), itraconazole (Sporanox) and mibefradil (Posicor) can cause marked increases in the amount of
sildenafil in the body. Patients taking these medications should be observed carefully if sildenafil is used.
Sildenafil increases the effects of the blood pressure lowering
medications. It also increases the blood pressure lowering effects of nitrates, e.g. isosorbide dinitrate (Isordil), isosorbide mononitrate (Imdur, Ismo, Monoket), nitroglycerin
(Nitro-Dur, Transderm-Nitro) that are used primarily for treating angina. Patients taking nitrates should not receive sildenafil. It is expected that rifampin will decrease blood
levels of sildenafil and probably reduce its effectiveness.

Sildenafil is used for the treatment of erectile dysfunction of either organic (medical condition) or psychogenic (psychological)
cause and for pulmonary arterial hypertension.