Skip to content
My WebMD Sign In, Sign Up

Dilantin Extended oral

Hydantoins/Folic Acid; Pyrimethamine

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:

When these two medicines are taken together, folic acid and pyrimethamine may cause your body to not process the hydantoin (e.g. phenytoin) properly.

What might happen:

Your blood level of the hydantoin may decrease which may cause an increased chance of seizures.

What you should do about this interaction:

The blood level of your hydantoin may need to be monitored more often. The dose of your hydantoin may need to be adjusted.Your healthcare professionals (e.g. doctor or pharmacist) 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.Kutt H, Winters W, McDowell FH. Depression of prahydroxylation of diphenylhydantoin by antituberculosis chemotherapy. Neurology 1966 Jun; 16(6):594-602.
2.Reynolds EH. Effects of folic acid on the mental state and fit-frequency of drug- treated epileptic patients. Lancet 1967 May 20;1(7499):1086-8.
3.Olesen OV, Jensen ON. The influence of folic acid on phenytoin (DPH) metabolism and the 24- hours fluctuation in urinary output of 5-(p-hydroxyphenyl)-5 phenyl- hydantoin (HPPH). Acta Pharmacol Toxicol (Copenh) 1970;28(4):265-9.
4.Jensen ON, Olesen OV. Subnormal serum folate due to anticonvulsive therapy. A double-blind study of the effect of folic acid treatment in patients with drug- induced subnormal serum folates. Arch Neurol 1970 Feb; 22(2):181-2.
5.Baylis EM, Crowley JM, Preece JM, Sylvester PE, Marks V. Influence of folic acid on blood-phenytoin levels. Lancet 1971 Jan 9;1(7689):62-4.
6.Mattson RH, Gallagher BB, Reynolds EH, Glass D. Folate therapy in epilepsy. A controlled study. Arch Neurol 1973 Aug;29(2):78-81.
7.Furlanut M, Benetello P, Avogaro A, Dainese R. Effects of folic acid on phenytoin kinetics in healthy subjects. Clin Pharmacol Ther 1978 Sep; 24(3):294-7.
8.Gibberd FB, Nicholls A, Wright MG. The influence of folic acid on the frequency of epileptic attacks. Eur J Clin Pharmacol 1981 Jan;19(1):57-60.
9.Inoue F. Clinical implications of anticonvulsant-induced folate deficiency. Clin Pharm 1982 Jul-Aug;1(4):372-3.
10.MacCosbe PE, Toomey K. Interaction of phenytoin and folic acid. Clin Pharm 1983 Jul-Aug;2(4):362-9.
11.Berg MJ, Fischer LJ, Rivey MP, Vern BA, Lantz RK, Schottelius DD. Phenytoin and folic acid interaction: a preliminary report. Ther Drug Monit 1983;5(4):389-94.
12.Berg MJ, Rivey MP, Vern BA, Fischer LJ, Schottelius DD. Phenytoin and folic acid: individualized drug-drug interaction. Ther Drug Monit 1983; 5(4):395-9.
13.Yuen GJ. Interaction of phenytoin and folic acid: an alternative explanation. Clin Pharm 1984 Mar-Apr;3(2):116, 119.
14.Rivey MP, Schottelius DD, Berg MJ. Phenytoin-folic acid: a review. Drug Intell Clin Pharm 1984 Apr;18(4):292-301.
15.Berg MJ, Fincham RW, Ebert BE, Schottelius DD. Phenytoin pharmacokinetics: before and after folic acid administration. Epilepsia 1992 Jul-Aug;33(4):712-20.

See 58 Reviews for this Drug. - OR -

Review this Treatment

Find a Drug:

by name or medical condition or shape/color (Pill Identifier)

(for example: aspirin)

(for example: diabetes)

Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.

Ask the pharmacist

Questions about medications? Get expert answers by video or live chat about allergies, pregnancy, sleep, and more.
See the Ask the Pharmacist event schedule.

Ask a Question

Popular Slideshows & Tools on WebMD

hands on abdomen
Can you catch one?
Woman sitting in front of UV lights
Is yours working?
woman using breath spray
What's causing yours?
womans hand on abdomen
Are you ready for baby?
MS Overview
Recognizing symptoms.
bowl of yogurt with heart shape
Eat for a healthy heart.
woman doing pushups
To help you get fit.
Colored x-ray of tooth decay
Know what to look for.
Woman sitting with child
Do you know the symptoms?
Stings, bites, burns, and more.
Allentown, PA
Are you living in one?
Thyroid exam
See how much you know.

Women's Health Newsletter

Find out what women really need.

WebMD the app

Get trusted health information. Whenever. Wherever... with your iPhone, iPad or Android.

Find Out More

IMPORTANT: About This Section and Other User-Generated Content on WebMD

The opinions expressed in WebMD User-generated content areas like communities, reviews, ratings, or blogs are solely those of the User, who may or may not have medical or scientific training. These opinions do not represent the opinions of WebMD. User-generated content areas are not reviewed by a WebMD physician or any member of the WebMD editorial staff for accuracy, balance, objectivity, or any other reason except for compliance with our Terms and Conditions. Some of these opinions may contain information about treatment or uses of drug products that have not been approved by the U.S. Food and Drug Administration. WebMD does not endorse any specific product, service, or treatment.

Do not consider WebMD User-generated content as medical advice. Never delay or disregard seeking professional medical advice from your doctor or other qualified healthcare provider because of something you have read on WebMD. You should always speak with your doctor before you start, stop, or change any prescribed part of your care plan or treatment. WebMD understands that reading individual, real-life experiences can be a helpful resource but it is never a substitute for professional medical advice, diagnosis, or treatment from a qualified health care provider. If you think you may have a medical emergency, call your doctor or dial 911 immediately.