Sildenafil citrate, sold as Viagra, Revatio and under various other trade names, is a drug used to treat erectile dysfunction and pulmonary arterial hypertension (PAH). It was developed and is being marketed by the pharmaceutical company Pfizer. It acts by inhibiting cGMP-specific phosphodiesterase type 5, an enzyme
that regulates blood flow in the penis. Since becoming available in
1998, sildenafil has been the prime treatment for erectile dysfunction;
its primary competitors on the market are tadalafil (Cialis) and vardenafil (Levitra).