Sildenafil prevents the breakdown of cycle guanosine monophosphate (cGMP) by inhibiting the activity of specific type 5 phosphodiesterases. When cGMP remains at higher levels, smooth muscle in the penis relaxes. Blood vessels dilate, which allows more blood to enter the cavernosa in the penis, filling them and causing an erection. After sexual stimulation, oral ED medications keep nitric oxide levels high by inhibiting the enzyme PDE5.