Is Calcium Carbonate (Antacid) Safe with phenytoin?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Do not take calcium carbonate antacids at the same time of day as phenytoin. Separate administration times to avoid reduced phenytoin absorption.
Why this interaction occurs
Calcium carbonate, as an antacid, may affect the absorption of phenytoin, potentially decreasing its serum levels. The FDA label specifically states that phenytoin and antacids should not be taken at the same time of day.
Evidence
Evidence level: strong.
Data source
FDA Drug Label
Reported safety data
The FDA’s adverse event reporting system (FAERS) has received 25,235 reports mentioning phenytoin. This reflects reported cases involving the medication in general — not confirmed causation, and not specific to the phenytoin + supplement combination on this page.
FAERS reports are voluntary and cannot prove that a drug caused an event. Do not use report counts to assess the safety of this specific drug-supplement pair.
Frequently asked questions
Related interactions
All Calcium Carbonate (Antacid) interactions → · All phenytoin interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.