Cholecalciferol × Hydrochlorothiazide

Thiazides reduce urinary calcium excretion: that property is exploited in calcium stone disease, but it means calcium is retained. If vitamin D simultaneously increases what comes in through the gut, the balance shifts towards hypercalcaemia. Risk rises with age, with summer dehydration and with reduced renal function.

Weakness, nausea, constipation, polyuria and thirst, confusion in the older patient. Moderate hypercalcaemia is easily mistaken for the diuretic's own effect or for dehydration.

This is not a forbidden combination: many patients with hypertension need vitamin D. What changes is the follow-up. Measure serum calcium before starting vitamin D and at 4–8 weeks, then periodically. Avoid high vitamin D doses without a documented deficiency and watch hydration in summer. If calcium rises, adjust the supplement first, not the antihypertensive.