Sildenafil can highly selectively inhibit the activity of PDE5 in the human body. PDE5 is highly expressed in the corpus cavernosum of the penis, but is less expressed in other tissues and organs of the human body. After taking sildenafil, the vascular smooth muscle of the corpus cavernosum of the penis relaxes under the action of the drug, blood flow increases, the corpus cavernosum becomes congested, and the penis becomes erect, thereby producing a therapeutic effect on erectile dysfunction.