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Some everyday medicines, herbal remedies, supplements and even foods such as grapefruit can make cancer drugs stronger and more toxic or weaker and less effective. Before treatment starts, and whenever anything changes, the team should check everything you take. Tell them about every tablet, remedy, vitamin and supplement, even ones you buy yourself.
Interactions with anticancer drugs are common and change both efficacy and toxicity. ASCO-ONS (2024) requires a medication list that includes over-the-counter medicines, herbal products and supplements, updated at every encounter and reviewed by a licensed practitioner when it changes, and pre-treatment education on interactions with prescribed drugs, integrative therapies, over-the-counter drugs and foods. It notes that patients may not mention herbal products or vitamins because they do not regard them as medicines, so staff must ask. BOPA (2025) makes the interaction check part of the pharmacist's verification at cycle 1 and for any new medicine at later cycles.
The interactions that matter most are pharmacokinetic: strong CYP3A inhibitors and inducers with oral targeted drugs (the ibrutinib label requires dose changes with inhibitors, avoidance of strong inducers, and no grapefruit or Seville oranges), and acid suppressants that cut absorption of pH-dependent kinase inhibitors (erlotinib: avoid proton pump inhibitors if possible, separate H2 blockers and antacids). St John's wort, a strong inducer, lowers levels of irinotecan's active metabolite and of imatinib. A Dutch multidisciplinary group graded 290 potential interactions: 94 clinically relevant with an advised intervention, 110 not relevant, and 86 with too little evidence; it argues for building such rules into electronic prescribing. In 202 older adults on anticancer treatment, 12% used herbs and 4% had a potentially relevant herb-drug interaction, half judged clinically relevant (red yeast rice, red coneflower, turmeric, cannabis). One widely named checker, Cancer Drug Interactions (Radboud UMC and University of Liverpool), has been closed since 2023, with an archive last updated in 2022.
Why: An interaction can only be found if every product the person takes is on the list.
“The patient's medication list inclusive of prescribed and over-the-counter medications, herbal products, and supplements is updated and documented in the medical record at every encounter and reviewed by a licensed practitioner when a change occurs”
Why: Many people do not mention remedies they buy themselves unless asked.
“Some patients, at times, may not disclose their use of herbal product, supplements, and vitamins during interviews because they may not regard these as medications. Pharmacists and providers must proactively inquire and document herbal product and complementary medications use within the medication list.”
Why: This is the point where a regimen can still be changed or another medicine swapped before harm is done.
“Check for potential interactions, contra-indications and duplication between all planned SACT & supportive medication and patient’s concurrent medication.”
Why: New prescriptions from other doctors, such as antibiotics, antifungals or antacids, are a common source of new interactions.
“Check patient allergy status, interactions or contraindications against any new medicines prescribed since last cycle (when information accessible).”
Why: Inhibitors raise blood levels and toxicity; inducers lower them and can make treatment fail.
“Avoid grapefruit and Seville oranges during IMBRUVICA treatment, as these contain strong or moderate inhibitors of CYP3A.”
Why: Lowering stomach acid can stop the cancer drug being absorbed, so less of it reaches the cancer.
“For proton pump inhibitors, avoid concomitant use if possible. For H-2 receptor antagonists and antacids, modify the dosing schedule”
Why: Most new medicines and supplements are started between clinic visits, often without the cancer team knowing.
“Planned duration of treatment, schedule of treatment administration, plan for missed doses, drug names and supportive medications, and drug interactions with prescribed drugs, integrative therapies, over the counter drugs, and foods”
Why: General checkers flag many interactions that do not matter and can miss ones that do; graded rules say which need action.
“Integration of DDI guidelines into the national electronic prescribing system is essential to achieve optimal efficacy and minimal toxicity in patients receiving anticancer therapy.”
Seminars in Oncology (van Leeuwen et al., Dutch Multidisciplinary Expert group) 2022
How care is usually carried out, read from the guidelines and standards cited. Your own team's protocol and their judgement come first; this page is for understanding it, not for doing it yourself.
A complete medication list including over-the-counter, herbal and supplement products, updated every visit and reviewed when it changes (ASCO-ONS 2024), with herbal use asked about directly; a pharmacist interaction check before cycle 1 and for each new medicine (BOPA 2025); label-directed management of CYP3A and acid-suppressant interactions; and patient education on interactions before treatment. No international guideline grades these steps; the only graded interaction-by-interaction guidance found is the Dutch multidisciplinary set (2022), read as an abstract.
ASCO and ONS (JCO Oncology Practice) 2024 · British Oncology Pharmacy Association 2025 · Seminars in Oncology (van Leeuwen et al., Dutch Multidisciplinary Expert group) 2022 · US National Library of Medicine, DailyMed 2026 · US National Library of Medicine, DailyMed 2024
Where it differs: A pharmacist reconciles medicines against the trial protocol's prohibited and monitored lists before enrolment.
Evidence: In 501 trial patients at one French centre, reconciliation found 346 interactions, 158 with prohibited medicines, more than half of them herbal, and 51% of prohibited or monitored drugs had been missed by oncologists (Malifarge 2024).
Where it differs: Uses a questionnaire on a set list of commonly used herbs with known interactions, then has two pharmacists judge each hit.
Evidence: In 202 adults aged 65 or over, herb use was 12% and potentially relevant herb-drug interactions 4%, half clinically relevant (Brokaar 2025).
Exploratory Research in Clinical and Social Pharmacy (Brokaar et al.) 2025
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