Reduced effects w/ anti-seizures (phenytoin, phenobarb, primidone), rifampicin & INH; glucocorticoids. Medications that cause fat malabsorption eg, orlistat, liqd paraffin, cholestyramine, can interfere w/ cholecalciferol absorption. Cytotoxic drugs, actinomycin & imidazole antifungal drugs interferes w/ the activity of vit D. Increased levels/effects of Al hydroxide, cardiac glycosides, Mg salts, sucralfate, vit D analogue.