Viagra is an exciting new drug that was introduced by Pfizer to treat impotence in men. The drug is a synthetic heterocyclic piperazine derivative that acts by selective inhibition of cyclic guanosine monophosphate (cGMP) phosphodiesterase type 5 allowing a relaxation of smooth muscles and inflow of blood to the corpus cavernosum, a condition that leads to erection during sexual stimulation. Because viagra serves to lower blood pressure in parts of the body it should not be taken in conjunction with nitrates, such as nitroglycerin, because in combination they can lower blood pressure too much. Side effects include headache, flushing, upset stomach, urinary tract infection, and diarrhea.