Vasoreactive pulmonary arterial hypertension (PAH) in children is a form of idiopathic PAH that responds to vasoreactive testing with nitric oxide (NO) by a significant decrease of pulmonary vascular resistances and pressure. Oral calcium channel antagonists (CCA) that allow pulmonary arterial vasodilation are the treatment of choice. The therapeutic effect is strongly depending on adequate drug intake. In growing children, drug dose must be adapted to weight. In case of unavailability of low-dose pharmaceutical preparations, officinal formulations become mandatory. Officinal formulations may be related to a multitude of errors at different steps including prescription, transcription, preparation and administration. This may have life-threatening consequences for the child.To illustrate this, we report a case of a compounding error with underdosage of CCA, leading to acute cardiovascular failure in an adolescent with vasoreactive PAH.