Sildenafil: an orally active type 5 cyclic GMP-specific phosphodiesterase inhibitor for the treatment of penile erectile dysfunction - Boolell,M.; Allen,M.J.; Ballard,S.A.; Muirhead,G.J.; Naylor,A.M.; Osterloh,I.H.; Gingell,J.C. (Int J Impotence Res, 1996)