Sildenafil
Lasts for up to 4 hours.
Taken about 60 minutes before sexual activity.
Onset of activity may be delayed with food.
Sexual stimulation required.
Tadalafil
Last for up to 36 hours.
Taken at least 30 minutes before sexual activity.
Can be taken with or without food.
Sexual stimulation required.
Vardenafil
Lasts for up to 5 hours.
Taken about 25 – 60 minutes before sexual activity.
Onset of activity may be delayed with a high fat meal.
Sexual stimulation required.