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 originally developed by British scientists and then brought to market by the US-based pharmaceutical company Pfizer. It acts by inhibiting cGMP-specific phosphodiesterase type 5 (PDE5) an enzyme that promotes degradation of cGMP which regulates blood flow in the penis.
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