Reduced effects w/ anti-seizures (eg, phenytoin, phenobarb, primidone), rifampicin & INH, glucocorticoids. Interference w/ cholecalciferol absorption w/ medications causing fat malabsorption (eg, orlistat, liqd paraffin, cholestyramine); vit D activity w/ cytotoxic drugs, actinomycin & imidazole antifungals. Increased levels/effects of Al hydroxide, cardiac glycosides, Mg salts, sucralfate, vit D analogue.