These medicines are all a bit different, however, they are all in the class called PDE-5 inhibitors. In short, what they do is increase the ability of blood vessels in the body to dilate. The blood vessels in the corpus cavernosum of the penis dilate and allow blood to fill the space creating an erection. PDE-5 inhibitors allow for both the creation of an erection as well as the maintenance of one. Where these medications differ is in strength and half-life. Half-life essentially refers to the amount of time the medicine is active in the system. Viagra (sildenafil) is generally considered the fastest acting of these meds but has a half-life of about 2-6 hours in most individuals. The strength of the medication more often leads to side effects than the other two. Cialis (tadalafil) is a longer-acting med but generally less intense with a half-life of 24-36 hours. Levitra (vardenafil) is a combination of the two in away. It has a shorter half-life than tadalafil but longer than sildenafil while acting closer to the intensity level of sildenafil.