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

Temozolomide (Temodar) is an oral chemotherapy drug for brain cancer. Like any medication — particularly a cytotoxic chemotherapy — it can interact with other drugs, supplements, and vaccines in ways that may increase side effects or reduce effectiveness. Here's what every patient and caregiver needs to know about temozolomide drug interactions before treatment begins.

Most Important: Tell Your Entire Care Team What You Take

Before starting temozolomide, tell your oncologist, pharmacist, and all other treating physicians about every medication you take — including prescription drugs, over-the-counter medications, vitamins, herbal supplements, and any vaccines you've had recently or are planning. This is especially important for GBM patients who are often on anti-seizure medications and steroids, both of which have interactions with temozolomide.

Major Interaction: Valproic Acid (Depakote, Depakene)

Valproic acid is an anti-seizure medication commonly used in brain tumor patients to prevent seizures. It has a clinically important interaction with temozolomide: valproic acid decreases the clearance of temozolomide by approximately 5%, slightly increasing temozolomide exposure (AUC). While this interaction is generally modest, it may increase temozolomide-related side effects, particularly myelosuppression. Tell your oncologist if you are on valproic acid so they can monitor your blood counts more closely.

Avoid: Live Vaccines During Treatment

Temozolomide suppresses the immune system. Live attenuated vaccines (including live influenza nasal spray, certain shingles vaccines, and others) should be avoided during temozolomide treatment because the immune suppression can allow the attenuated (weakened) live virus to cause disease. Additionally, temozolomide may reduce the immune response to vaccines in general, making them less effective. Inactivated vaccines (the injectable flu shot, pneumococcal vaccine, etc.) are still recommended but should be discussed with your oncologist about timing.

Avoid: Drugs That Cause Additive Myelosuppression

Temozolomide lowers white blood cell counts and platelets. Combining it with other drugs that also suppress bone marrow can lead to dangerously low counts. Medications that require extra caution or avoidance include:

• Deferiprone: An iron chelating agent associated with agranulocytosis. Combining with temozolomide greatly increases the risk of severe neutropenia. Avoid if possible; if unavoidable, monitor absolute neutrophil count very closely.
• Ropeginterferon alfa-2b: This medication is a myelosuppressive agent; combining it with temozolomide can cause additive bone marrow suppression. Avoid combination use.
• Acalabrutinib (Calquence) and other BTK inhibitors: May increase myelosuppressive effects when combined with temozolomide. Monitor blood counts closely.

Caution: Immunosuppressive Drugs

Temozolomide itself has immunosuppressive effects. Combining it with other immunosuppressive agents can increase the risk of serious and opportunistic infections. Relevant categories include:

• Tofacitinib (Xeljanz): Increased immunosuppression and infection risk.
• Etrasimod: Immunosuppressive effects may be additive; avoid combination.
• Belatacept: Additive immunosuppressive effects increase infection risk.

Important: Carbamazepine, Phenytoin, and TMP-SMX in Context

Carbamazepine (Tegretol) and phenytoin (Dilantin) are anti-seizure medications sometimes used in brain tumor patients. The prescribing information notes that patients who experienced aplastic anemia while on temozolomide had concomitant exposure to carbamazepine, phenytoin, or sulfamethoxazole/trimethoprim — all drugs associated with aplastic anemia on their own. This complicates assessing causality, but warrants extra monitoring of CBC in patients on these combinations.

Palifermin: Timing Interaction

Palifermin (Kepivance) is used to prevent oral mucositis in some patients receiving chemotherapy. It should not be administered within 24 hours before, during, or after temozolomide administration — coadministration increased the severity and duration of mucositis in clinical studies.

Drug Interactions and MGMT Inhibitors: What About Lomustine?

Lomustine (CCNU) is sometimes combined with temozolomide in recurrent GBM protocols. This combination is intentional — it provides complementary DNA-damaging mechanisms — but it also produces additive myelosuppression. This combination requires very careful CBC monitoring and is only undertaken in specialized oncology settings.

What to Tell Every Doctor and Pharmacist

• That you are taking temozolomide — including the dose and schedule
• All anti-seizure medications you are taking (phenytoin, carbamazepine, levetiracetam, valproic acid, lacosamide, etc.)
• All corticosteroids (dexamethasone) — very commonly used in brain tumor patients
• Any supplements, including immune-boosting supplements, that could affect your blood counts or immune function
• Any planned vaccines — your oncologist should approve vaccine timing during chemotherapy

For more on side effects to watch for during treatment, see: Temozolomide Side Effects: What to Expect and When to Call Your Doctor.

If you need help finding your temozolomide prescription, medfinder.com can locate pharmacies near you that have it in stock.