Fildena 25 mg contains sildenafil citrate, which is a medication primarily used to treat erectile dysfunction (ED) in men. It belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors. When a man is sexually stimulated, nitric oxide is released into the erectile tissue of the penis, which activates the enzyme guanylate cyclase. This enzyme increases levels of cyclic guanosine monophosphate (cGMP), which relaxes the smooth muscles in the blood vessels of the penis, allowing for increased blood flow and thus, an erection.