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

Emend (aprepitant) has a complex drug interaction profile due to its effects on the cytochrome P450 enzyme system—specifically CYP3A4 and CYP2C9. Before starting Emend, your care team needs a complete list of every medication, supplement, and herbal product you take. This article explains the most important interactions and what they mean for your treatment.

Why Emend Interacts With So Many Drugs

Emend is metabolized in the liver primarily by an enzyme called CYP3A4. This same enzyme is responsible for breaking down hundreds of other medications. Emend acts as a moderate inhibitor of CYP3A4 during its short course—meaning it slows down how fast the body processes other CYP3A4-metabolized drugs, temporarily raising their blood levels.

After the short course ends, Emend also induces (speeds up) CYP3A4 and CYP2C9—the opposite effect—which can reduce the levels of certain other drugs.

The bottom line: Emend can raise or lower the levels of many drugs in your blood, depending on the specific medication and timing. Drug interactions with Emend can be serious. Disclosing all your medications is not optional—it's essential.

Contraindicated Drugs: Never Use With Emend

Pimozide (Orap): Used for Tourette syndrome. Emend significantly increases pimozide blood levels via CYP3A4 inhibition, which can cause dangerous QT prolongation (a heart rhythm abnormality) that may lead to life-threatening arrhythmias. This combination is absolutely contraindicated.

Major Interactions: Proceed With Caution

Warfarin (Coumadin): Emend induces CYP2C9, which metabolizes warfarin. This decreases warfarin levels and lowers the INR (the measure of how well your blood is clotting)—potentially increasing your risk of blood clots. INR must be monitored 7-10 days after each aprepitant course.

Hormonal contraceptives (pills, patches, rings, implants, injections): Emend reduces the effectiveness of hormonal birth control by inducing the enzyme that breaks down hormonal contraceptives. Use a non-hormonal backup method (condom, diaphragm, spermicide) during treatment and for 28 days after your last dose.

Rifampin (a strong CYP3A4 inducer): Rifampin dramatically reduces aprepitant blood levels—by approximately 11-fold—making Emend much less effective. Avoid combining these unless directed by your oncologist.

Ketoconazole and other strong CYP3A4 inhibitors: These antifungal medications slow down Emend's metabolism, significantly increasing aprepitant blood levels and increasing the risk of Emend's side effects.

Ifosfamide (a chemotherapy agent): Aprepitant can increase ifosfamide-induced neurotoxicity by inducing CYP3A4, which alters the metabolism of ifosfamide toward neurotoxic metabolites. Behavioral changes and confusion have been reported; close monitoring is required.

Moderate Interactions: Dose Adjustment May Be Needed

Dexamethasone: Emend increases dexamethasone blood levels by about 50% (oral) or 25% (IV). Your oncologist will reduce the dexamethasone dose by 50% or 25% accordingly when Emend is in your protocol.

Benzodiazepines (midazolam, alprazolam, triazolam): Emend increases blood levels of benzodiazepines metabolized by CYP3A4, potentially increasing sedation, confusion, and respiratory depression.

Diltiazem (a calcium channel blocker): Mutual interaction: diltiazem increases aprepitant levels approximately 2-fold, and aprepitant also increases diltiazem levels. This can affect heart rate and blood pressure. Blood pressure and heart rate should be monitored.

Tolbutamide (a diabetes medication): A CYP2C9 substrate. Emend may reduce tolbutamide levels, potentially affecting blood sugar control.

Certain CYP3A4-metabolized cancer drugs: Several chemotherapy agents including cyclophosphamide, docetaxel, erlotinib, etoposide, imatinib, irinotecan, and vinblastine are CYP3A4 substrates. Aprepitant may alter their blood levels; your oncologist monitors for this.

Natural Supplements That Interact With Emend

St. John's Wort: A powerful CYP3A4 inducer. Taking St. John's Wort with Emend can dramatically reduce aprepitant blood levels, making it much less effective. Avoid St. John's Wort entirely while on chemotherapy.

What to Tell Your Doctor Before Starting Emend

Before your first Emend dose, give your oncologist and pharmacist a complete list of:

All prescription medications (including blood thinners, heart medications, antibiotics, antifungals, HIV medications, and psychiatric medications)

All over-the-counter medications (pain relievers, antacids, cold remedies)

All vitamins and mineral supplements

All herbal supplements (especially St. John's Wort, echinacea, ginkgo, ginseng)

Your method of birth control (critical—see hormonal contraceptive interaction above)

How Your Healthcare Team Manages These Interactions

Your oncology pharmacist plays a crucial role in reviewing your medication list for interactions before every chemotherapy cycle. Dose adjustments of dexamethasone and other medications are built into standard Emend-containing protocols. For a review of Emend's side effects alongside its interactions, see our article on Emend side effects: what to expect and when to call your doctor.

And if you need help locating Emend at a pharmacy near you before your next cycle, medfinder can do the searching for you.