Women receiving oral contraceptives have shown a significant increase in plasma vitamin A level. Excessive dosage of vitamin D induces hypercalcaemia and in some instances hypercalciuria. Hypercalcaemia in patients receiving digitalis may precipitate cardiac arrhythmias.
Cholestyramine has been reported to reduce absorption of fat soluble vitamins: as such it may impair intestinal absorption of vitamin A and vitamin D.
The administration of anti-convulsants has been shown to affect the vitamin D requirements in some patients.
Pyridoxine may decrease the efficacy of levodopa in the treatment of parkinsonism. It should be used with caution for patients undergoing levodopa therapy.
Ascorbic acid taken concomitantly with fluphenazine may necessitate an increase in dosage of fluphenazine.
The absorption of both iron salts and tetracyclines is diminished when they are taken concomitantly by mouth. If treatment with both drugs is required, a time interval of about 2 to 3 hours should be allowed between each administration. The absorption of iron salts may also be decreased by some antacids. The response to iron may be delayed in patients receiving concomitant chloramphenicol therapy. Iron salts have been reported to decrease absorption and thus reduce the bioavailability and clinical effect of levodopa with carbidopa, methyldopa and penicillamine; decreased absorption and bioavailability has also been reported for the fluoroquinolones ciprofloxacin and ofloxacin when administrated with iron salts.
Administration of corticosteroids may intefere with calcium absorption.
Calcium enhances the effects of digitalis on the heart and may precipitate digitalis intoxication. Calcium salts reduce the absorption of tetracyclines.