category of drugs that relieve erectile dysfunction ( impotence) in men. Two common commercially produced PDE-5 inhibitors are sildenafil (sold as Viagra) and vardenafil (Levitra). PDE-5 inhibitors work by blocking, or inhibiting, the action of phosphodiesterase-5 (PDE-5), an enzyme naturally present in the corpus cavernosum, the spongy erectile tissue of the penis. Under normal circumstances, sexual arousal in the male stimulates neurons in the corpus cavernosum to release nitric oxide, a chemical compound that causes the formation of cyclic guanosine monophosphate (cGMP); cGMP in turn causes the smooth muscle of the corpus cavernosum to relax, allowing blood to flow into the penis and produce an erection. PDE-5 breaks down cGMP, and so the PDE-5 inhibitors, by blocking the action of the enzyme, maintain higher levels of cGMP and preserve a satisfactory erection.

PDE-5 inhibitors are prescribed as oral drugs to be taken approximately one hour before sexual intercourse. Because they can also inhibit other biologically active forms, or isoenzymes, of phosphodiesterase (e.g., PDE-6, PDE-11), they have been known to produce side effects such as facial flushing, headache, and blurry or coloured vision. They are not prescribed for men who take nitrate drugs for angina pectoris, as chemical interactions between the two medications may cause a dangerous drop in blood pressure. Also, PDE-5 inhibitors are not effective against impotence caused by mental disorders such as severe depression or by physical conditions such as nerve damage. Nevertheless, for millions of men they have revolutionized the treatment of common erectile dysfunction, which previously was treated with surgical implants, suppositories, pumplike devices, or injection of drugs directly into the penis.