apotheke jarrestrasse What Is Cialis? What Is Tadalafil?Tadalafil, known under the brand name Cialis for the treatment of erectile dysfunction and Adcirca for pulmonary arterial hypertension, is a PDE5 inhibitor. Its main competitors are sildenafil citrate (Viagra) and vardenafil (Levitra). Tadalafil's chemical formula is C22H19N3O4. Cialis became nicknamed "the weekend pill" because its effects last 36 hours