Sildenafil (Viagra). This medication is most effective when taken on an empty stomach one hour before sex. It's effective for four to five hours or more if you have mild to moderate erectile dysfunction. Vardenafil (Levitra, Staxyn).

Sildenafil is a prescription drug. It comes in the form of a tablet and suspension (liquid). Both are taken by mouth. It also comes in an intravenous (IV) form, which is only given by a doctor.

Viagra (sildenafil) is a prescription medication that treats erectile dysfunction (ED). Viagra is an oral medication. It’s available as 25 mg, 50 mg, and 100 mg tablets. You can take a dose about 1 hour before sexual activity. Viagra is available as a lower-cost generic.