Fildena 50 Sildenafil citrate is a PDE5 inhibitor, which blocks the action of the enzyme PDE 5 and improves blood circulation in the penis. This increases blood flow to the penis and helps men achieve a firm erection for sexual stimulation. It also increases a chemical in the blood called cGMP, which relaxes the muscles in the penis and expands the blood vessels. You should not use this medicine if you have a heart problem, low blood pressure, or a stroke. You should also avoid drinking alcohol while taking this medicine. You should also not take this medicine if you are pregnant or planning to become pregnant.