A compound of formula I ##STR1## or a pharmaceutically acceptable salt thereof in which R.sub.1 and R.sub.2 are independently H or methyl (for example N,N-dimethyl-1-[1-4chlorophenyl)cyclobutyl]-3-methylbutyl amine hydrochloride optionally in the form of its monohydrate) is used for treating pulmonary hypertension, particularly in patients who take certain anorectic medications.