Tadalafil. Molecular model of the drug tadalafil (C22.H19.N3.O4), a PDE5 (phosphodiesterase type 5) inhibitor used to treat erectile dysfunction and pulmonary arterial hypertension. It is marketed under the trade names Cialis and Adcirca. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue) and oxygen (red).