In vivo, erection is mediated by neural stimulation that is nonadrenergic and noncholinergic. However, papaverine induced erections were found to result in priapism, pain, heamatoma by Cara et. Drugs can also inhibit erectile functioning locally. Medical conditions, including cardiovascular disease and diabetes can lead to erectile problems in some individuals. This would explain why individuals on blood thinners may have difficulty maintaining erections.
Clinical studies are reasonably impressive.
A large n clinical trial was reported by Goldstein et. This study employed 532 men with erectile dysfunction of organic, psychogenic and mixed causes. This in turn was followed in 329 men in a 32 week, open label extension study. Note the less than universal success rate. The authors conclude that oral sildenafil is an effective, well-tolerated treatment for erectile dysfunction.