Reduced plasma conc w/ phenytoin, phenobarb, primidone & other enzyme inducers. Calcifediol can cause hyperglycemia, which can enhance the inotropic effects of digoxin & its toxicity, producing cardiac arrhythmias. Decreased absorption w/ cholestyramine, colestipol or orlistat. Decreased intestinal absorption w/ paraffin & mineral oil. Co-administration of hydrochlorothiazide w/ vit D supplements in patients w/ hypoparathyroidism may lead to hypercalcemia. Some antibiotics (eg, penicillin, neomycin, chloramphenicol) can increase Ca absorption. Hypermagnesemia may occur w/ phosphate-binding agents (eg, Mg salts) since vit D has an effect on phosphate transport in the intestine, kidney & bone. Potential inhibition of antianginal action of verapamil. Additive effects & hypercalcemia can occur w/ co-administration of any vit D analogue. Avoid uncontrolled intake of additional Ca-containing prep. Corticosteroids counteract the effects of vit D analogue drugs. Additive effects may occur w/ vit-D-supplemented food.