Sildenafil is a phosphodiesterase type 5 (PDE‑5) inhibitor. During sexual arousal, your body releases nitric oxide (NO), which increases cGMP levels in the penis. Sildenafil prevents the breakdown of cGMP, allowing for smooth muscle relaxation in penile blood vessels, increased blood flow, and a stronger erection. It requires sexual stimulation to be effective and does not cause spontaneous erections DailyMed+3