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.
Tadalafil is a PDE5 inhibitor, currently marketed in pill form for treating erectile dysfunction (ED) under the name Cialis; and under the name Adcirca for the treatment of pulmonary arterial hypertension.
Vardenafil is used to treat male sexual function problems (impotence or erectile dysfunction) by blocking a certain enzyme (phosphodiesterase-PDE5). In combination with sexual stimulation, vardenafil works by helping the blood flow into the penis to achieve and maintain an erection.