Cialis (tadalafil) & Viagra (sildenafil) are said to be oral drugs that are used for treating ED (the inability to attain or maintain a penile erection) & benign prostatic hyperplasia (BPH) in men. They are in a group of drugs known as phosphodiesterase-5 (PDE5) inhibitors that also involve vardenafil (Levitra, Staxyn ODT), and avanafil (Stendra).
Moreover, The erection of the penile is caused by the engorgement of the penile area with blood. Under normal situations, sexual stimulation leads to the formation & release of nitric oxide in the penile area. Nitric oxide then activates the enzyme, guanylate cyclase, which later causes the formation of cyclic guanosine monophosphate (cGMP).