Tadalafil [Cialis] is an oral drug that is used to cure impotency in men. It belongs to a group of drugs called phosphodiesterase inhibitors. Tadalafil was approved by the FDA in November 2003. By disallowing muscle relaxation in the penis and increased blood flow, phosphodiesterase type 5 [PDE5] reduces the erectile functioning. Tadalafil sets this dysfunction right by inhibiting PDE5.