Sildenafil citrate works by increasing blood flow to the penis during sexual stimulation, helping men achieve and maintain an erection. It does this by inhibiting the enzyme phosphodiesterase type 5, which normally breaks down cyclic guanosine monophosphate (cGMP), a substance that promotes relaxation of smooth muscles and dilation of blood vessels in the penis. By inhibiting this enzyme, sildenafil citrate helps to prolong the effects of cGMP, leading to increased blood flow and improved erectile function.