Drug	Standard dose*	Recommended time between dosing and intercourse	Onset of action	Duration†
Sildenafil (Viagra)	50 to 100 mg	One hour	14 to 60 minutes	Up to four hours
Tadalafil (Cialis)	10 to 20 mg	One to 12 hours	16 to 45 minutes	Up to 36 hours
Vardenafil (Levitra)	10 to 20 mg	One hour	25 minutes	Up to four hours