The effect of vitamin D can be decreased by concomitant treatment with phenytoin, barbiturates or corticosteroids because of changes in metabolic activation.
Effects of digoxin and other cardiac glycosides may be accentuated by oral administration of calcium plus vitamin D:
strict medical supervision is needed in patients on this combination and, if necessary, monitoring of ECG and calcium concentrations when using vitamin D supplementation in these situations
There is an increased risk of hypercalcemia when receiving thiazide diuretics, as they can reduce urinary calcium excretion.
Calcium salts may reduce absorption of levothyroxine, bisphosphonates, sodium fluoride, quinolone or tetracycline antibacterials or iron :
it is advisable to allow 2-4 hours before taking a calcium-containing vitamin D supplement when on such medication
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