Why Viagra doesn't always work.
D004334:Drug Administration Schedule; D007172:Erectile Dysfunction / Q000188:drug therapy; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010879:Piperazines / Q000008:administration & dosage*; D017698:Postmenopause; D011687:Purines; D012737:Sex Factors; D012735:Sexual Dysfunction, Physiological / Q000188:drug therapy*; D000068677:Sildenafil Citrate; D013450:Sulfones; D017211:Treatment Failure; D014665:Vasodilator Agents / Q000008:administration & dosage*
D010879:Piperazines; D011687:Purines; D013450:Sulfones; D014665:Vasodilator Agents; D000068677:Sildenafil Citrate