Polymedication is frequent in geriatric everyday practice and often leads to unwanted drug-drug interactions. Physicians must be aware of the potential interaction-induced with intoxications or therapeutic ineffectiveness. The clinical case presented here illustrates the importance of entertaining the possibility of such an interaction every time that the expected therapeutical effect of a substance is not reached. Our patient had refractory arterial hypertension despite taking five different antihypertensive drugs in hospital, while on carbamazepine for a facial neuralgia. This substance induces the cytochrome P450, which catalyse the metabolism of most of the antihypertensives used. After the discontinuation of carbamazepine and on tritherapy, the blood pressure profile has significantly improved.