This invention provides for a unit dosage form of treprostinil at a dosage of between 0.1 mg/mL and 25 mg/mL in a sterile fluid composition formulated for subcutaneous or intravenous injection, which composition does not comprise an antimicrobial preservative. Selectively activatable patch-pump assemblies comprising a sealed prefilled drug-reservoir containing the unit dosage form is described as are methods for reducing pain at a site of subcutaneous or intravenous infusion of treprostinil in a subject in need thereof, and methods of reducing irritation, inflammation or a combination thereof at a site of subcutaneous or intravenous infusion of treprostinil in a subject in need thereof. The invention also provides a method of treating pulmonary hypertension, and methods for treating, or reducing the severity or reducing the pathogenesis of Pulmonary arterial hypertension (PAH) in a subject making use of the compositions as herein described.
