Easy. Sildenafil for the use of ED is patented under the name Viagra and in it's specific doses(25mg, 50mg, 100mg). Sildenafil for the use of treating pulmonary hypertension is NOT patented so it is offered by generic companies in 20mg doses. Same med, different use and dose = generic US "viagra".
It is the same thing with say Finasteride. 1mg = Propecia which is used for male pattern baldness, and 5mg = Proscar which is used for BPH/Enlarged prostate.
Here is Revatio(sildenafil), which is used for pulmonary hypertension.
Prices and Coupons for Revatio