Sildenafil treats erectile dysfunction by selectively inhibiting phosphodiesterase-5 (PDE5), an enzyme that breaks down cyclic guanosine monophosphate (cGMP) in the corpus cavernosum. Increased cGMP levels lead to relaxation of smooth muscle and vasodilation, allowing enhanced blood flow into the penis and facilitating erection. This mechanism is dependent on nitric oxide release during sexual stimulation but does not initiate it directly. Therefore, sildenafil enhances the natural erectile response rather than causing spontaneous erections.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, Brunton et al., 13th Edition.