Which statement describes phenytoin interactions?

Which statement describes phenytoin interactions?

• A. Decreased absorption of calcium and vitamin D
• B. Increases the effect of anticoagulants and oral contraceptives
• C. Increases absorption of calcium
• D. No interactions

Answer: (A)

Phenytoin acts as a powerful inducer of hepatic enzymes and, over time, disrupts vitamin D metabolism. This leads to lower active vitamin D levels, which reduces calcium absorption from the gut. With less calcium available and ongoing bone turnover, bone mineral density can decline, increasing the risk of osteoporosis. That’s why decreased absorption of calcium and vitamin D is the best description of phenytoin’s interactions in this context. It's helpful to keep in mind that phenytoin can also reduce the effectiveness of other drugs such as anticoagulants and oral contraceptives by speeding up their metabolism, rather than increasing their effects. And phenytoin does have meaningful interactions, so a statement of no interactions isn’t accurate.

Phenytoin acts as a powerful inducer of hepatic enzymes and, over time, disrupts vitamin D metabolism. This leads to lower active vitamin D levels, which reduces calcium absorption from the gut. With less calcium available and ongoing bone turnover, bone mineral density can decline, increasing the risk of osteoporosis. That’s why decreased absorption of calcium and vitamin D is the best description of phenytoin’s interactions in this context.

It's helpful to keep in mind that phenytoin can also reduce the effectiveness of other drugs such as anticoagulants and oral contraceptives by speeding up their metabolism, rather than increasing their effects. And phenytoin does have meaningful interactions, so a statement of no interactions isn’t accurate.