7 Imatinib Interactions to Watch Out for: Warfarin, St. John’s Wort, and More

Cancer medications have come a long way: They can help slow down cancer growth, improve survival odds, and even put people in remission in some cases. But while these medications are important tools for fighting cancer, a bit of risk management is needed. Managing drug interactions is one aspect of successful treatment.

Imatinib (Gleevec) is an oral, prescription-only medication that treats many types of cancer. It can interact with medications used to treat health conditions like abnormal heart rhythms, seizures, high cholesterol, and more. These interactions can affect imatinib’s safety and effectiveness. In some cases, they can also impact how well other medications work.

Here, we review some medications — as well as supplements and foods — to be wary of while taking imatinib.

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Imatinib interactions at a glance

Imatinib can interact with medications that affect specific liver enzymes (proteins). One example is CYP3A4, which is responsible for breaking down many medications.

Some medications and substances, called CYP3A4 inducers, can accelerate how fast the protein works. This can lead to lower imatinib levels in the blood and less effective cancer treatment. Some substances that have this effects are:

• St. John's wort

• Phenytoin (Dilantin)

• Oxcarbazepine (Trileptal)

• Rifampin (Rifadin)

• Carbamazepine (Tegretol)

CYP3A4 inhibitors have the opposite effect. They make it harder for the protein to do its job, which slows down imatinib’s breakdown. This increases the risk of side effects from imatinib. Examples of substances that have this effect include:

• Ritonavir (Norvir) and ritonavir-containing medications like Paxlovid

• Nelfinavir (Viracept)

• Ketoconazole

• Itraconazole (Sporanox)

• Fluconazole (Diflucan)

• Posaconazole (Noxafil)

• Voriconazole (Vfend)

• Cobicistat (Tybost)

• Erythromycin (Ery-Tab)

• Clarithromycin (Biaxin XL)

• Diltiazem (Cardizem, Cartia)

• Verapamil (Verelan)

• Grapefruit juice

Imatinib can have a similar effect on other medications. Imatinib may increase blood levels of medications that are broken down by CYP3A4 and/or the protein CYP2D6. Here are just a few of the medications that it can affect in this way:

• Atorvastatin (Lipitor)

• Rosuvastatin (Crestor)

• Simvastatin (Zocor)

• Amlodipine (Norvasc)

• Nifedipine (Procardia, Procardia XL)

• Nisoldipine (Sular)

• Warfarin (Coumadin, Jantoven)

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This list of interacting medications isn’t comprehensive. Speak to your cancer care team if you’re concerned about potential interactions with imatinib. It’s hard to keep track of interactions on your own.

Here, we’ll dive deeper into seven notable imatinib interactions.

1. Certain statins

Statins are prescription medications that lower cholesterol. Some examples you may be familiar with are atorvastatin (Lipitor), simvastatin (Zocor), and rosuvastatin (Crestor).

Imatinib can interfere with the liver enzymes that are in charge of metabolizing statins, causing statin levels to accumulate in the blood. Simvastatin is particularly sensitive to this effect. Levels of simvastatin can double or triple if you’re also taking imatinib. This interaction may cause you to experience new or worsening side effects, including muscle pain. 

Let your cancer care team know if you’re taking a statin. They may want to lower your statin dosage so that you can safely take both medications together. Or they may switch you to a statin that doesn’t interact as strongly with imatinib.

2. Calcium channel blockers

Amlodipine (Norvasc) is a commonly prescribed calcium channel blocker (CCB) that you may be familiar with. It treats high blood pressure, angina (chest pain), and heart disease. 

Imatinib interacts with amlodipine and several other CCBs because it slows down how quickly the liver processes these medications. This can cause higher CCB blood levels and increase the risk of side effects, including hypotension (blood pressure that’s too low).

Two CCBs, diltiazem and verapamil, have a different effect. These medications can slow down how quickly the liver breaks down imatinib. This can increase imatinib levels in the blood and raises the risk of side effects such as nausea, muscle cramps, and diarrhea.

To avoid a potential interaction, your prescriber may decide to proactively adjust your imatinib or CCB dosage out of caution.

3. Warfarin

Warfarin (Coumadin, Jantoven) is a prescription-only blood thinner that prevents and treats blood clots. It’s a narrow therapeutic index drug, which means blood levels of the blood thinner should be tightly maintained. These levels shouldn’t go up or down too quickly.

Warfarin is broken down by two different liver enzymes. Imatinib can slow down both of these enzymes. This interaction can cause warfarin levels to rise in the blood, which increases the risk of bleeding.

Your cancer care team will likely conduct routine lab work while you’re taking imatinib, especially if you’re also taking a blood thinner. However, they’ll still likely recommend switching from warfarin to a different blood thinner, such as enoxaparin (Lovenox), that doesn’t interact with imatinib.

4. Anti-seizure medications

Phenytoin and carbamazepine are prescription-only medications that are used to manage seizures. They frequently interact with other medications by making liver enzymes work faster.

If you take phenytoin with imatinib, imatinib levels in your blood can go down. Imatinib is less effective at fighting cancer when these levels drop too much. Carbamazepine, oxcarbazepine, and other anti-seizure medications have a similar effect.

Your healthcare professional may help you manage this potential interaction by proactively increasing your imatinib dosage. Alternatively, they may work with your neurologist to adjust your anti-seizure medication regimen so it won’t drastically decrease imatinib levels. This is done on a case-by-case basis.

5. St. John’s wort

St. John’s wort is an over-the-counter dietary supplement that people sometimes take for depression. It can interact with several medications, though, including imatinib. 

St. John’s wort can cause liver enzymes to work faster. This, again, can translate to lower imatinib levels and, potentially, a less-effective treatment. 

It’s best to avoid St. John’s wort while taking imatinib, if possible. Speak to a healthcare professional if you’re having depression symptoms. They can recommend other treatments that are safer to pair with imatinib, such as cognitive behavioral therapy or prescription antidepressants.

6. Certain antifungals and antivirals

Some antifungals and antivirals slow down liver enzymes, too. This includes ketoconazole, ritonavir, and more. Blood levels of imatinib can rise when it’s taken with these medications. This can lead to new or worsening imatinib side effects.

This interaction doesn’t apply to every medication in these groups. So, if you need an antifungal or antiviral to treat an infection, your healthcare professional will likely prescribe you one that has a low risk of interacting with imatinib. Alternatively, they may adjust your imatinib dosage.

7. Grapefruit juice

Grapefruit juice may seem like a harmless and even healthy addition to your diet. However, it interacts with many medications, including imatinib. 

Grapefruit juice can slow down the speed at which liver enzymes break down imatinib, especially when consumed in large quantities. This can lead to higher imatinib levels in the blood and an increased risk of side effects. 

To be safe, it’s best to avoid consuming grapefruit juice while you’re taking imatinib. You may want to opt for other types of fruit juice, such as orange juice or cranberry juice. Water, sparkling or still, is also a great alternative.

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Written by:

Mandeep Sohal, PharmD

Mandeep Sohal, PharmD, has more than 7 years of experience in the pharmaceutical industry and is licensed in California. In his career, he’s had experience with specialty pharmacy, community pharmacy, pharmacy benefit management, and pharmaceutical manufacturers.

Edited by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

Reviewed by:

Vanessa Carranza, PharmD

Vanessa Carranza, PharmD, is a pharmacist who has dedicated her career to the advancement of medical education for healthcare providers, patients, and caregivers, most notably in the oncology space.

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