SulfonesPurinesPiperazinesPhosphodiesterase 5 InhibitorsPhosphodiesterase InhibitorsErectile Dysfunction3',5'-Cyclic-GMP PhosphodiesterasesCyclic Nucleotide Phosphodiesterases, Type 5Penile ErectionVasodilator AgentsCyclic GMPHomosexuality, MaleMethamphetamineGeorgiaUnsafe SexRisk-TakingSexual BehaviorHypertension, PulmonaryCarbolinesImpotence, VasculogenicPenisPhosphoric Diester HydrolasesPyrimidinonesAdministration, OralTriazinesPulmonary ArteryPriapismCross-Over StudiesDouble-Blind Method