Phenytoin/Aluminum-Magnesium Hydroxide; Oral Calcium Interactions

This information is generalized and not intended as specific medical advice. Consult your healthcare professional before taking or discontinuing any drug or commencing any course of treatment.

Medical warning:

Moderate. These medicines may cause some risk when taken together. Contact your healthcare professional (e.g. doctor or pharmacist) for more information.

How the interaction occurs:

Aluminum hydroxide, magnesium hydroxide, and calcium may bind to phenytoin in your digestive tract, preventing your body from absorbing your phenytoin.

What might happen:

The amount of phenytoin in your blood may decrease and it may not work as well.

What you should do about this interaction:

Let your healthcare professionals (e.g. doctor or pharmacist) know that you are taking these medicines together. Your doctor may want to check the amount of phenytoin in your blood. Separate the times you take your phenytoin and your antacid or medicine that contains calcium by as much time as possible. Let your doctor know right away if you have any changes in the amount of seizures you have.Your healthcare professionals may already be aware of this interaction and may be monitoring you for it. Do not start, stop, or change the dosage of any medicine before checking with them first.

  • 1.Dilantin (phenytoin sodium) US prescribing information. Pfizer, Inc. March, 2022.
  • 2.Garnett WR, Carter BL, Pellock JM. Effect of calcium and antacids on phenytoin bioavailability. Arch Neurol 1980 Jul;37(7):467.
  • 3.Carter BL, Garnett WR, Pellock JM, Stratton MA, Howell JR. Effect of antacids on phenytoin bioavailability. Ther Drug Monit 1981;3(4):333-40.
  • 4.Kulshrestha VK, Thomas M, Wadsworth J, Richens A. Interaction between phenytoin and antacids. Br J Clin Pharmacol 1978 Aug;6(2):177-9.

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CONDITIONS OF USE: The information in this database is intended to supplement, not substitute for, the expertise and judgment of healthcare professionals. The information is not intended to cover all possible uses, directions, precautions, drug interactions or adverse effects, nor should it be construed to indicate that use of a particular drug is safe, appropriate or effective for you or anyone else. A healthcare professional should be consulted before taking any drug, changing any diet or commencing or discontinuing any course of treatment.