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The oral men's drug sildenafil, sold under the brand name Viagra, has been a media sensation. Since its introduction into America in April 1998 there has been unprecedented demand and it earned its manufacturers, Pfizer, global revenues of about $900 million during 1998. The diamond-shaped blue tablet, Viagra is taken about 60 minutes before sexual activity. Sildenafil works by inhibiting an enzyme called 5-phosphodiesterase, thus increasing the levels a substance called cyclic GMP. Cyclic GMP is a powerful dilator of blood vessels in the penis, leading to inflow of blood and an erection under stimulating circumstances.