Could taking phentermine alone cause pulmonary hypertension?

Phentermine. While it is uncertain, a prospective surveillance study1 of 579 patients—205 with primary pulmonary hypertension and 374 with secondary pulmonary hypertension—showed no relation between primary pulmonary hypertension and Phentermine (odds ratio 0·6, 95% ci 0·2—2·2).
Yes. Pulmonary hypertension occurs after consumption of appetite suppressing drugs such as Fenfluramine & Dexfenfluramine which have been commonly prescribed with phentermine. When used alone, it can still cause pulmonary hypertension & a lot of other serious side effects. Please consult your doctor to discuss the risks versus benefits before taking any medication.
Yes. It has been associated with higher incidence of pulmonary hypertension but it is dependent on how long you are on it and certain predisposing genetic factors.