Drug, Food and Supplement Interactions — With Trastuzumab Deruxtecan

Paracetamol at standard doses does not significantly interact with trastuzumab deruxtecan through the CYP3A4 pathway and is commonly used for pain and fever during cancer treatment. Ibuprofen and other NSAIDs are a separate consideration — they can affect kidney function and platelet behaviour, both of which may already be altered by treatment. Ask your oncology team before taking either one regularly, and always mention the dose and how often you are taking it.

A single small amount is unlikely to cause a serious problem, but the interaction is real. Grapefruit blocks the CYP3A4 enzyme for a significant period — you cannot clear it quickly by drinking water. Tell your team it happened, particularly if you notice any new or worsening side effects in the days that follow. The prescribing information advises avoiding it completely throughout treatment, not only around infusion days.

Fluconazole is a moderate CYP3A4 inhibitor and can raise levels of the active drug payload. This does not make it automatically unsafe, but your oncologist should know before you start it. In many cases your team can suggest a topical antifungal that works locally in the mouth without the same systemic effect. Do not stop fluconazole already prescribed without advice — but call your oncology team the same day to let them know it has been prescribed.

Do not stop carbamazepine on your own. Anti-seizure medicines must only be changed under your neurologist's guidance, and stopping suddenly can be dangerous. Carbamazepine is a strong CYP3A4 inducer and can lower the level of active drug in your body. Your oncologist and neurologist need to discuss whether an alternative seizure medicine with less enzyme activity would be appropriate, or whether the treatment plan needs to account for the interaction. This is a conversation between specialists.

Eating turmeric as a spice in food is very different from taking concentrated curcumin capsules, which deliver far higher amounts. Ashwagandha has been reported to have some effect on CYP enzymes, though its interaction with trastuzumab deruxtecan specifically has not been well studied. We do not yet have enough data to say these are safe at supplemental doses during this treatment. Tell your oncologist what you are taking and at what dose, and follow their advice rather than deciding alone.

Most standard blood pressure medicines do not interact with trastuzumab deruxtecan through the CYP3A4 pathway and are continued during treatment. Heart medicines are a separate consideration because trastuzumab deruxtecan can affect the heart, and your team will monitor cardiac function during treatment. Tell your oncologist the names and doses of every heart and blood pressure medicine you take before your first cycle, so they can factor them into your monitoring plan.

Warfarin is metabolised through a different pathway, but trastuzumab deruxtecan can independently affect platelet counts and bleeding risk. The combination means your team will want to monitor you more closely rather than avoid it automatically. If you are on warfarin, make sure both your oncologist and your anticoagulation team know. Your INR may need more frequent checking in the early cycles, and any new bruising, bleeding gums, or blood in urine or stool should be reported the same day.

Yes — this is one of the most useful things you can do. Include prescription medicines, over-the-counter tablets and syrups, vitamins, minerals, protein powders, herbal capsules, teas and any injections you receive outside the hospital. Write down the name, the dose and how often you take each one. If a product only has a brand name on the packet, bring the packet. Your team may not act on every item, but they cannot advise you on what is safe without knowing what is there.