The introduction of the well-known drug Viagra® (sildenafil) on the market involved a huge improvement in the treatment of erectile dysfunction. During sexual arousal, large amounts of NO are formed in the endothelial cells in the corpus cavernosum of the penis. The NO stimulates the formation of cGMP in the smooth muscle cells, which induces muscle relaxation by decreasing the intracellular calcium concentration.



The drug sildenafil is a selective inhibitor of the enzyme phosphodiesterase-5 (PDE-5), thereby maintaining high cGMP levels and thus erection.

Compare this mechanism of action with that of nitrates in vasodilation.

The adverse effects of sildenafil result from the vasodilating mechanism and include headache, flushing and dizziness.


Sildenafil should not be combined with which of the following drugs?