Dilantin Drug Interactions: What to Avoid and What to Tell Your Doctor

Dilantin (phenytoin) has one of the most complex drug interaction profiles of any commonly prescribed medication. It both affects and is affected by dozens of other drugs — through multiple mechanisms involving liver enzymes, protein binding, and more. Understanding these interactions is essential for your safety.

Why Does Dilantin Have So Many Drug Interactions?

There are two reasons phenytoin interacts with so many drugs:

Phenytoin itself is heavily metabolized by enzymes. It is broken down primarily by CYP2C9 and CYP2C19 liver enzymes. Any drug that inhibits these enzymes will cause phenytoin levels to rise (increasing toxicity risk). Any drug that induces these enzymes will cause phenytoin levels to fall (risking breakthrough seizures).

Phenytoin is a potent enzyme inducer. It powerfully activates CYP3A4, the enzyme that metabolizes roughly half of all drugs. This speeds up the breakdown of those drugs — reducing their effectiveness at standard doses.

Drugs That INCREASE Phenytoin Levels (Risk of Toxicity)

These drugs inhibit the enzymes that break down phenytoin, causing blood levels to rise — which can cause nystagmus, ataxia, confusion, and cardiac effects:

Antifungals (azoles): Fluconazole (Diflucan), ketoconazole, voriconazole — significantly increase phenytoin levels. If you need antifungal treatment, your doctor should check levels more frequently.

Amiodarone (heart drug): A commonly used antiarrhythmic that significantly raises phenytoin levels. Combination requires very close monitoring.

Antibiotics: Metronidazole (Flagyl), isoniazid (TB antibiotic), cotrimoxazole/trimethoprim-sulfamethoxazole (Bactrim), and chloramphenicol all increase phenytoin levels.

Valproate/Divalproex (Depakote): A complex interaction — valproate competes with phenytoin for protein binding, which can initially increase free (active) phenytoin while total levels may appear unchanged or low. Both drugs affect each other; close monitoring is required.

Cimetidine (Tagamet): An older heartburn medication that inhibits phenytoin metabolism. Omeprazole and other PPIs are preferred.

Disulfiram (Antabuse): Used to treat alcohol dependence; can significantly increase phenytoin levels.

Drugs That DECREASE Phenytoin Levels (Risk of Breakthrough Seizures)

These drugs induce the enzymes that break down phenytoin, causing levels to fall — which can lead to breakthrough seizures:

Carbamazepine (Tegretol): Induces phenytoin metabolism (while phenytoin also induces carbamazepine metabolism — a bidirectional interaction)

Rifampin: A powerful enzyme inducer used to treat tuberculosis that significantly lowers phenytoin levels

Barbiturates: Chronic use can decrease phenytoin levels; acute use may increase them

Theophylline: Used for asthma; can lower phenytoin levels

Drugs AFFECTED by Dilantin (Phenytoin Reduces Their Effectiveness)

Because phenytoin is a potent CYP3A4 inducer, it reduces the blood levels of many other important medications:

Hormonal contraceptives (birth control pills, patches, rings): Phenytoin significantly reduces contraceptive hormone levels — making hormonal birth control unreliable. Women on phenytoin should use a non-hormonal backup method (condom, IUD). This is critical: unintended pregnancy while taking Dilantin carries serious fetal risk.

Direct oral anticoagulants (apixaban/Eliquis, edoxaban/Savaysa): Phenytoin significantly reduces anticoagulant levels, potentially leading to treatment failure and increased clotting risk. These combinations are contraindicated or require careful alternative management.

HIV antiretrovirals: Atazanavir, dolutegravir, cabotegravir, and others are significantly reduced by phenytoin induction. This can lead to treatment failure and HIV drug resistance.

Corticosteroids: Phenytoin can reduce steroid blood levels, potentially reducing effectiveness of anti-inflammatory or immunosuppressive therapy

Digoxin: Phenytoin can reduce digoxin levels in the blood

Many cancer drugs: Phenytoin is a strong CYP3A4 inducer and reduces the effectiveness of many targeted cancer therapies including abemaciclib, everolimus, dronedarone, and others

Food and Supplement Interactions

Antacids (calcium, magnesium, aluminum): Reduce phenytoin absorption. Take Dilantin 2–3 hours before or after antacids.

Tube feeding (enteral nutrition): Continuous tube feeding can significantly reduce phenytoin absorption. Levels should be monitored carefully in patients receiving tube feeding.

Alcohol: Chronic heavy alcohol use lowers phenytoin levels (enzyme induction). Acute heavy alcohol use can unpredictably affect levels. Alcohol also lowers the seizure threshold.

Folic acid supplements: Folate can reduce phenytoin levels. Take as directed by your doctor, but be aware of this interaction.

What to Tell Every Healthcare Provider and Pharmacist

Because phenytoin's interactions are so extensive, every time you start or stop any medication — prescription, over-the-counter, herbal, or supplement — you should inform:

Your neurologist or prescribing doctor

Your pharmacist (who can run a drug interaction check)

Any new prescriber — including dentists, emergency room doctors, or specialists

For more on Dilantin safety, see our guide on Dilantin Side Effects: What to Expect and When to Call Your Doctor.