Pulmonary arterial hypertension (PAH) is a rare condition in which the small branches of the pulmonary arteries, known as arterioles, experience wall thickening and constriction. These pathological changes raise the pressure in the pulmonary arteries, as well as cause increased enlargement and strain on the right ventricle. People with PAH experience progressive shortness of breath, and nearly always need to use extra oxygen. Several classes of medication are used to treat PAH, including prostanoids such as iloprost, endothelin receptor antagonists like bosentan, and phosphodiesterase type 5 (PDE5) inhibitors. This last group of agents includes sildenafil, which is marketed under the trade name Revatio, but is identical to the medication Viagra, used to treat male erecticle dysfunction. Tadalafil (Adcirca) is the newest member of the PDE5 inhibitors, which has a long half-life in the circulation, and only needs to be taken once a day.