Effect of calcium and antacids on phenytoin bioavailability

The effects of bihourly administration of gluconate calcium and tow magnesium-aluminum-containing acids on the bioavailability of a single capsule dose of phenytoin sodium were determined in two normal volunteers. Neither the rate nor the extent of phenytoin absorption were altered by the treatments, in spite of indirect evidence from other studies that might suggest such an interaction. No interaction between calcium or magnesium and either unionized or anionic phenytoin could be demonstrated in vitro using solubility and spectral techniques.