Sildenafil inhibits PDE5 by binding to it. As you have noted, brand-name Viagra sildenafil is in the same ballpark. Sildenafil Viagra Sildenafil, more commonly known as Viagra, was not intended to induce erection. A lot of studies have been made on the efficacy of this erectile dysfunction drug. With more GMP available, the blood vessels dilate, so more blood comes in. Its action is similar to other PDE5 inhibitors, such as tadalafil and vardenafil, which we will discuss later.Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash. Blink Health states that vardenafil, the generic form of Levitra, starts at $ for 30 tablets of 20 mg. As of September , generic daily 5 mg tadalafil can cost around $8 per dose, and higher doses of generic tadalafil — 10 mg and 20 mg — can start around $44 per pill. Pros. It can be taken.