Since calcitriol is one of the most important active metabolites of vitamin D3, pharmacological doses of vitamin D and its derivatives should be withheld during treatment with calcitriol to avoid possible additive effects and hypercalcemia. Dietary instructions, especially calcium supplements should be strictly observed, and uncontrolled intake of additional calcium-containing preparations must be avoided.
Concomitant treatment with a thiazide diuretic increases the risk of hypercalcemia. Calcitriol dosage must be determined with care in patients undergoing treatment with digitalis, as hypercalcemia in such patients may precipitate cardiac arrhythmias.
A relationship of functional antagonism exists between vitamin D analogues, which promote calcium absorption, and corticosteroids, which inhibit it.
Magnesium-containing drugs (eg, antacids) may cause hypermagnesemia and should therefore not be taken during therapy with calcitriol by patients on chronic renal dialysis. Since calcitriol also has an effect on phosphate transport in the intestine, kidneys and bones, the dosage of phosphate binding agents must be adjusted in accordance with the serum phosphate concentration (normal values: 2.5 mg/100 mL or 0.6-1.6 mmol/L).
Administration of enzyme inducers eg, phenytoin or phenobarbital may lead to increase metabolism and, hence, reduced serum concentrations of calcitriol. Therefore, higher doses of calcitriol may be necessary if these drugs are administered simultaneously.
Cholestyramine can reduce intestinal absorption of fat-soluble vitamins and therefore may impair intestinal absorption of calcitriol.