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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:

The cause of the interaction is not known. When these two medicines are taken together, your body may not process primidone properly.

What might happen:

Primidone is converted by your body to phenobarbital. Your blood levels of phenobarbital may increase and cause toxic effects.

What you should do about this interaction:

Contact your doctor as soon as possible if you experience confusion, difficulty sleeping, difficulty breathing, very slow breathing, fainting, or rash. Contact your healthcare professionals (e.g. doctor or pharmacist) if you experience continued or bothersome dizziness, drowsiness, lightheadedness, clumsiness, or excessive daytime drowsiness.Your healthcare professionals may already be aware of this drug interaction and may be monitoring you for it. If your doctor prescribes these medicines together, it may be necessary to check the phenobarbital levels in your blood more often. Do not start, stop, or change the dosage of any medicine before checking with them first.


1.Gallagher BB, Baumel IP, Mattson RH, Woodbury SG. Primidone, diphenylhydantoin and phenobarbital. Aspects of acute and chronic toxicity. Neurology 1973 Feb;23(2):145-9.

2.Wilson JT, Wilkinson GR. Chronic and severe phenobarbital intoxication in a child treated with primidone and diphenylhydantoin. J Pediatr 1973 Sep; 83(3):484-9.

3.Fincham RW, Schottelius DD, Sahs AL. The influence of diphenylhydantoin on primidone metabolism. Arch Neurol 1974 Mar;30(3):259-62.

4.Schmidt D. The effect of phenytoin and ethosuximide on primidone metabolism in patients with epilepsy. J Neurol 1975 Jun 9;209(2):115-23.

5.Reynolds EH, Fenton G, Fenwick P, Johnson AL, Laundy M. Interaction of phenytoin and primidone. Br Med J 1975 Jun 14;2(5971):594-5.

6.Callaghan N, Feely M, Duggan F, O'Callaghan M, Seldrup J. The effect of anticonvulsant drugs which induce liver microsomal enzymes on derived and ingested phenobarbitone levels. Acta Neurol Scand 1977 Jul;56(1):1-6.

7.Garrettson LK, Gomez M. Phenytoin-primidone interaction. Br J Clin Pharmacol 1977;4:693-5.

8.Lambie DG, Johnson RH. The effects of phenytoin on phenobarbitone and primidone metabolism. J Neurol Neurosurg Psychiatry 1981 Feb;44(2):148-51.

9.Porro MG, Kupferberg HJ, Porter RJ, Theodore WH, Newmark ME. Phenytoin: an inhibitor and inducer of primidone metabolism in an epileptic patient. Br J Clin Pharmacol 1982 Aug;14(2):294-7.

Selected from data included with permission and copyrighted by First Databank, Inc. This copyrighted material has been downloaded from a licensed data provider and is not for distribution, expect as may be authorized by the applicable terms of use.

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.

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