The active ingredient in Kamagra 100mg  is sildenafil citrate, which is a PDE5 (phosphodiesterase type 5) inhibitor. Erectile dysfunction occurs due to constriction of the arteries/blood vessels that restrict blood flow to the p*nis. Sildenafil citrate inhibits the PDE5 responsible for this constriction and, at the same time, increases the release of nitric oxide (NO), which causes dilation of blood vessels in the corpus cavernosum. It does this by protecting cGMP (cyclic guanosine monophosphate) from being broken down.