1- The absorption of vitamin D is improved by calcium, choline, fats, phosphorus, and vitamins A and C.
2- Rifampin, H2 blockers, barbiturates, heparin, isoniazid, colestipol, cholestyramine, carbamazepine, phenytoin, fosphenytoin, and phenobarbital reduce serum levels of vitamin D and increase its metabolism.
3- Overuse of mineral oil and stimulant laxatives may deplete vitamin D.
4- Osteoporosis and hypocalcemia can result from prolonged use of corticosteroids. It is necessary to take of calcium and vitamin D together with corticosteroid drugs.