Sildenafil citrate works by inhibiting the enzyme phosphodiesterase type 5 (PDE5). This inhibition leads to increased levels of cyclic guanosine monophosphate (cGMP) in the smooth muscle cells of the penis. Elevated cGMP levels relax smooth muscle and dilate blood vessels, resulting in increased blood flow to the penis, which helps achieve and maintain an erection in response to sexual stimulation.