Supplements and phenytoin: folic acid, vitamin D and antacids

October 20, 2026By VitOptics Staff

Short answer: Phenytoin (Dilantin, Phenytek) has a long list of nonprescription interactions, and several of them run in both directions. MedlinePlus lists folic acid, vitamin D, antacids that contain calcium, magnesium or aluminum, and St. John's wort among the products that may interact with it. The Dilantin label says vitamins such as folic acid can alter phenytoin levels, and that phenytoin may lower vitamin D levels. Talk with your prescriber or pharmacist before you add or drop any of these. MedlinePlus is clear on the medicine itself: "Do not stop taking phenytoin without talking to your doctor, even if you experience side effects such as unusual changes in behavior or mood."

This guide explains why these particular products come up, what the label says about each, and what to ask at your next appointment or refill.

Why phenytoin works both ways with supplements

Most interaction lists run in one direction: a supplement affects a medicine. Phenytoin's label has two lists.

Things that can change phenytoin levels. The label's table of drugs that may decrease phenytoin levels includes folic acid, St. John's wort and antacids (calcium carbonate, aluminum hydroxide and magnesium hydroxide). Salicylates and the acid reducers cimetidine and omeprazole appear on its list of drugs that may increase them.
Things phenytoin can change. The label lists folic acid among drugs whose level is decreased by phenytoin, and vitamin D among drugs whose efficacy is impaired by it.

The label also explains why interactions matter with this medicine. Phenytoin is broken down in the liver by an enzyme system that can become saturated, so the label says it is particularly susceptible to interactions that slow that breakdown, which may produce significant increases in phenytoin levels. The label says that adding or withdrawing an interacting product "may require an adjustment of the phenytoin dose," which is a decision for your prescriber. MedlinePlus puts it plainly for patients: "Do not start any of these medications while taking phenytoin without discussing with your healthcare provider."

Folic acid, including multivitamins and B-complex

Folic acid is a B-complex vitamin the body needs to make red blood cells. It is also the clearest example of a two-way interaction with phenytoin: the label lists folic acid as something that may lower phenytoin levels, and phenytoin as something that lowers folic acid levels. In its patient counseling section, the label tells prescribers to inform patients that vitamins such as folic acid "can alter their phenytoin levels."

Folic acid is not only sold on its own, so read the Supplement Facts panel of every multivitamin, B-complex or blend you take, and check every label for repeated ingredients. Because the label talks about both adding and withdrawing interacting products, stopping a multivitamin that contains folic acid is also a change worth telling your prescriber about. Whether you need folic acid, and how much, is something your prescriber decides with you.

Vitamin D, calcium and what the label says about bone

The Dilantin label has a section titled "Effects on Vitamin D and Bone." It says long-term use of phenytoin in people with epilepsy has been associated with decreased bone mineral density (osteopenia, osteoporosis and osteomalacia) and bone fractures. It explains that phenytoin induces (increases the activity of) liver enzymes, which "may enhance the metabolism of vitamin D and decrease vitamin D levels, which may lead to vitamin D deficiency, hypocalcemia, and hypophosphatemia" (low vitamin D, low calcium and low phosphate). The label tells prescribers that consideration should be given to screening with bone-related laboratory and radiological tests as appropriate.

MedlinePlus lists vitamin D among the nonprescription products that may interact with phenytoin, and asks you to tell your doctor if you have or have ever had osteopenia, osteomalacia or osteoporosis. So vitamin D and calcium are reasonable topics to raise, but the amount, the form and whether you need them at all are questions for your prescriber, ideally with recent blood test results in front of you.

Antacids and acid reducers on the list

MedlinePlus names antacids that contain calcium, magnesium or aluminum (Maalox, Mylanta, Tums and others), plus the acid reducers cimetidine (Tagamet), famotidine (Pepcid), nizatidine (Axid) and ranitidine (Zantac). The label says antacids may affect how phenytoin is absorbed, and its management note for antacids reads: "Phenytoin and antacids should not be taken at the same time of day." Ask your pharmacist how that applies to the products you use.

One overlap is easy to miss. MedlinePlus describes calcium carbonate as both a dietary supplement for people who don't get enough calcium from food and an antacid for heartburn. If your calcium supplement is calcium carbonate, ask your pharmacist whether the label's antacid note covers it too.

MedlinePlus also lists salicylate pain relievers, including aspirin and magnesium salicylate (Doan's). If you take any of these regularly, include them on your list.

St. John's wort

St. John's wort is on both the MedlinePlus list and the label, which says it may decrease phenytoin levels and that its effect on liver enzymes "may vary widely based on preparation." The National Center for Complementary and Integrative Health says St. John's wort can weaken the effects of many medicines, and names phenytoin and carbamazepine among them. NCCIH's advice applies to anyone taking a medicine: talk with your health care provider before using St. John's wort or other herbal products.

If you take a different anti-seizure medicine, its list may look different. MedlinePlus lists St. John's wort for carbamazepine, and aspirin and cannabidiol (CBD) for valproic acid. Ask your pharmacist what applies to yours. If you take gabapentin (Neurontin), our guide to gabapentin and supplements covers its antacid timing and the questions about sleep aids.

What to ask (level checks when your supplements change)

MedlinePlus lists phenytoin among the most common medicines checked with therapeutic drug monitoring, a blood test of the medicine's level. It says to tell your provider about other medicines or supplements you take, in case they affect the results. The Dilantin label recommends checking phenytoin levels when an interaction is suspected, and says those checks are best made after a change has had time to settle rather than right away. MedlinePlus also says to keep all appointments with your doctor and the laboratory, and to tell the laboratory staff you take phenytoin before any test.

Useful questions:

I take, or plan to take, a product with folic acid. Should my phenytoin level be checked after I start or stop it?
Has my vitamin D been checked, and are bone-related tests appropriate for me?
Is a vitamin D or calcium supplement right for me, and how should it fit with my phenytoin schedule?
Which antacid or heartburn medicine should I use, if any?
Is anything else I take, including herbal products, on my medicine's list?

If you or someone you care for receives formula or supplements through a feeding tube, MedlinePlus says to talk to your doctor about when to take phenytoin. Bring a written list of everything you take, with labels or photos; our guide on what to review before starting a new supplement walks through what to include.

Where VitOptics fits

VitOptics builds personalized vitamin packs from a short questionnaire, and the free assessment asks which prescription medicines you take, so you can list phenytoin by name. Its health screen includes an option for "Seizure disorder, bipolar disorder, or high-risk psychiatric medication." If you tick it because of a seizure disorder or a seizure medicine such as phenytoin, your recommendation goes to a person on our team before anything is released. Automated checks review the recommendation; some profiles need additional review by a person on our team. A second automated check reads the whole pack before release. Your result lists every formula, the amount of each ingredient and the reason it is there, so you can see whether any folic acid, vitamin D, calcium or magnesium is included and show it to your prescriber or pharmacist before you start. Our comparison of personalized packs and a multivitamin covers the difference.

Whichever product you choose, talk with your prescriber or pharmacist about any supplement while you take phenytoin. Each VitOptics pack also names one time of day for the whole pouch, which is worth showing your pharmacist next to your phenytoin schedule.

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Sources

This article is for educational purposes and is not medical advice. Supplements are not intended to diagnose, treat, cure or prevent any disease.

Prepared with AI assistance using the sources linked in this article.

Last updated: October 2026

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