Sildenafil is an inhibitor of the enzyme phosphodiesterase-5. Phosphodiesterase causes a breakdown of the neurotransmitter cGMP. This substance causes a slackening of the muscles at the erectile tissue of the penis and promotes the inflow of blood. Thus, if PDE-5 is inhibited, the rate of degradation of cGMP decreases (levels of cGMP remain elevated) and the erection can be better maintained.
Sildenafil is rapidly absorbed from the gastrointestinal tract and reaches its maximum blood concentration 30-60 minutes after ingestion. Sildenafil is approximately 96% protein bound and has a half-life of approximately 3-5 hours. Sildenafil is broken down in the cells of the liver by enzymes of the CYP450 family. It is then mostly excreted in the stool.
It may interact with drugs that are also metabolized by the CYP450 enzyme system. These include ketoconazole, itraconazole, sequinavir, erythromycin, and cimetidine. The potent CYP3A4 inhibitor ritonavir may result in excessive plasma levels of sildenafil. When taken concomitantly, life-threatening side effects are the possible consequence.