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

When you start Temodar (temozolomide), one of the most important conversations to have with your oncologist is about drug interactions. As a chemotherapy drug, temozolomide interacts with a number of other medications — some in ways that increase toxicity risk, others in ways that reduce the drug's effectiveness. Here is a comprehensive look at what matters in 2026.

Tell Your Doctor About ALL Your Medications

Before starting temozolomide, give your oncologist a complete list of every medication you take — including:

Prescription medications (including all doses)

Over-the-counter drugs (aspirin, ibuprofen, acetaminophen, antacids, etc.)

Vitamins and mineral supplements

Herbal supplements (St. John's Wort, turmeric, etc.)

Any recreational substances or cannabis products

Key Drug Interactions to Know

1. Valproic Acid (Depakote, Depakene)

Valproic acid is an antiepileptic drug commonly prescribed to brain tumor patients to prevent seizures — so many temozolomide patients are taking it. Research shows that valproic acid decreases the clearance of temozolomide by approximately 5%, meaning it stays in the body slightly longer at higher concentrations. This can increase the risk of temozolomide's side effects, particularly myelosuppression.

Your oncologist should be aware if you are taking valproic acid. This combination is not automatically contraindicated but requires monitoring. Some oncologists may prefer alternate antiepileptics (like levetiracetam/Keppra) that have fewer interactions.

2. Live Vaccines

Temozolomide significantly suppresses the immune system. Receiving any live attenuated vaccine during treatment is generally contraindicated and potentially dangerous — the weakened virus or bacteria in the vaccine could cause actual infection in an immunocompromised patient. Examples of live vaccines include measles-mumps-rubella (MMR), varicella (chickenpox), yellow fever, and nasal flu vaccine (FluMist).

Inactivated vaccines (like the injectable flu shot, pneumococcal, and COVID-19 vaccines) are not live and can generally be given, though immune response may be reduced during chemotherapy. Discuss vaccination timing with your oncologist.

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3. Other Immunosuppressants

Combining temozolomide with other drugs that suppress the immune system can compound the risk of serious infections. This includes corticosteroids (like dexamethasone, commonly used to reduce brain tumor swelling), other chemotherapy agents, and biological agents used to prevent organ rejection. Your oncologist will carefully weigh these combinations, but it's important to disclose all immunosuppressant medications.

4. Carbamazepine, Phenytoin, Sulfamethoxazole/Trimethoprim

These medications are specifically called out in the Temodar prescribing information because they are independently associated with aplastic anemia — the same serious blood condition that temozolomide can cause. Using them together complicates assessment of whether aplastic anemia is being caused by the drug or by temozolomide. Notify your oncologist if you're taking carbamazepine (Tegretol), phenytoin (Dilantin), or sulfamethoxazole/trimethoprim (Bactrim/Septra).

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5. Dacarbazine (DTIC)

Temozolomide is actually contraindicated in patients with a known hypersensitivity to dacarbazine (DTIC) because both drugs are metabolized to the same active compound (MTIC). If a patient has had an allergic reaction to dacarbazine in the past, they may have a cross-reactive allergy to temozolomide. This is an absolute contraindication.

6. Ranitidine (Zantac)

Clinical studies have shown that ranitidine (an H2 blocker antacid medication) does not significantly change the pharmacokinetics (absorption, distribution, or clearance) of temozolomide. This is a minor interaction and generally not a clinical concern.

Antiemetics and Drug Interactions

Many patients take anti-nausea medications (antiemetics) with temozolomide. Prochlorperazine (Compazine) and ondansetron (Zofran) are commonly used and generally have not shown clinically significant interactions with temozolomide. However, some antiemetics can cause constipation or QT prolongation — always discuss your antiemetic regimen with your oncologist.

Temodar Drug Interaction Quick Reference Table

Valproic acid — Moderate interaction; increases TMZ exposure by ~5%; monitor for increased toxicity

Live vaccines — Major interaction; avoid during immunosuppressive chemotherapy

Dacarbazine — Contraindicated in patients with dacarbazine hypersensitivity (cross-reactivity risk)

Carbamazepine, phenytoin, TMP/SMX — Moderate; complicate aplastic anemia risk assessment; report to oncologist

Immunosuppressants/corticosteroids — Additive immunosuppression; increased infection risk — manage carefully

Ranitidine — Minor; no clinically significant change in TMZ pharmacokinetics

Also read: Temodar Side Effects: What to Expect and When to Call Your Doctor

If you're having trouble finding your temozolomide prescription, medfinder can help locate which pharmacies near you can fill it.