Sildenafil is a medicine in a group called PDE5 inhibitors. In tablets it is supplied as sildenafil citrate, a salt form that helps it dissolve. The strength on the pack, 50mg or 100mg, is the amount of sildenafil itself.
It works by relaxing the smooth muscle in the blood vessels of the penis, so more blood can flow in when you are sexually aroused. It does not cause an erection on its own, and it does not increase sexual desire.
Sildenafil is taken only when needed, not every day. In Malaysia it is a prescription medicine, sold as the originator brand Viagra and as registered generics such as Iqnyde.