When the company learned that erections were one of the drug's side effects, it changed course and began developing the drug as a treatment for impotence. As Dr. Huang Shih-tsung, an attending physician in Chang Gung Memorial Hospital's urology department, has written, Viagra doesn't itself cause the corpus cavernosum to fill with blood.
長庚醫院泌尿外科主治醫師黃世聰為文指出，威而鋼的作用機轉，只是延長男性生殖器海綿體內「環狀鳥甘一磷酸」的作用時間，並不能直接刺激海綿體充血勃起。