MODERATE — Monitor Closely
TIMING — separate administration by at least 2 hours
Evidence level: MODERATE
Taking calcium at the same time as phenytoin reduces how much phenytoin your body absorbs.
Calcium can reduce phenytoin absorption if taken simultaneously by forming insoluble complexes in the GI tract.
Take calcium supplements at least 2 hours before or after phenytoin. Do not take with calcium-fortified foods at the same time.
Important: calcium, magnesium, and iron can chelate with some antiepileptics (especially phenytoin) and reduce absorption. Space Calcium at least 2-3 hours from Phenytoin. Long-term antiepileptic use can deplete calcium and vitamin D — supplementation may be medically appropriate.
Simultaneous administration, high-dose calcium supplementation
Seizure breakthrough if phenytoin levels drop, low phenytoin blood levels
Discuss timing of calcium supplementation relative to phenytoin doses.
Antiepileptic drug levels are sensitive to enzyme inducers and inhibitors. Discuss any changes with your neurologist before adding new substances. Safer supplement options: magnesium, omega-3s, and B vitamins are generally well-tolerated alongside seizure medications.
Taking calcium at the same time as phenytoin reduces how much phenytoin your body absorbs.
Important: calcium, magnesium, and iron can chelate with some antiepileptics (especially phenytoin) and reduce absorption. Space Calcium at least 2-3 hours from Phenytoin. Long-term antiepileptic use can deplete calcium and vitamin D — supplementation may be medically appropriate.
Seizure breakthrough if phenytoin levels drop, low phenytoin blood levels
Antiepileptic drug levels are sensitive to enzyme inducers and inhibitors. Discuss any changes with your neurologist before adding new substances. Safer supplement options: magnesium, omega-3s, and B vitamins are generally well-tolerated alongside seizure medications.
Discuss timing of calcium supplementation relative to phenytoin doses.
Or browse the full interaction database (121,000+ pairs).