Sildenafil, an orally active type 5 cGMP-specific phosphodiesterase inhibitor for the treatment of erectile dysfunction - Boolell, M; Allen, MJ; Ballard, SA; Gepi-Altee, S; Murhead, GJ; Naylor, AM; Osterloh, IH; Gingell, C (Int J Impot Res, 1996)