Sildenafil (also known as Viagra), vardenafil (also known as Levitra), and tadalafil (known as Cialis
here) all work by inhibiting PDE5, Cialis's pharmacologic distinction is its longer half-life (18 hours) — compared to Viagra (3–5 hours) and Levitra (3–6 hours) — resulting in longer duration of action, and so responsible for "The Weekend Pill" sobriquet. Weekend Pill is a nickname of Cialis pill. Furthermore, the longer half-life is the basis for current investigation of tadalafil's daily therapeutic use in relieving pulmonary arterial hypertension.