Sildenafil is a phosphodiesterase‑5 inhibitor used to treat erectile dysfunction and to improve exercise capacity in pulmonary arterial hypertension, working by enhancing nitric‑oxide–cGMP signaling to relax smooth muscle and increase targeted blood flow in the corpus cavernosum and pulmonary vasculature. It requires sexual stimulation to be effective for erections and does not increase sexual desire or prevent sexually transmitted infections. Common brand names include Viagra for erectile concerns and Revatio for pulmonary hypertension.