Silagra 100 mg contain sildenafil, a phosphodiesterase type 5 (PDE5) inhibitor that acts on the animal tissue of the penis to extend blood flow, which is required to cause an erection. During sexual stimulation gas is released within the animal tissue of the penis which activates the enzyme guanylate cyclase. This enzyme increases levels of a chemical called cyclic guanosine monophosphate (cGMP), which relaxes the blood vessels within the penis and allows blood to fill the spongy erectile tissues to cause an erection.