Medical Oncologist
Before you start
Which of Your Existing Medicines — Must Stop Before Targeted Therapy
Some of the medicines you take every day can interfere with how a targeted therapy drug works — either by reducing its effect or by raising it to a level that causes side effects. Knowing which ones to declare, and how to do it, is a straightforward step you can take before treatment begins.
Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed August 2026
- Not just prescription drugs — Herbal supplements, vitamins, and over-the-counter products can all affect how a targeted drug is absorbed or broken down by your body.
- Some interactions change the drug level in your blood — A few common medicines can lower your targeted drug to a level where it stops working, or raise it until side effects appear.
- A written list is more reliable than memory — Your oncologist cannot review what they do not know about. Bringing a written list to your appointment removes the risk of forgetting something in clinic.
- Do not stop medicines without being told to — Stopping a regular medicine suddenly — especially for the heart, blood pressure, or diabetes — can cause its own harm. The plan needs to come from your team.
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Before targeted therapy starts, tell your oncologist about every medicine you take — including herbal supplements, vitamins, and over-the-counter products. Some, like St John's Wort and antacids, are known to affect how targeted drugs work. Your team will advise which to stop and when, based on the specific drug you are starting.
Why do some of your regular medicines interfere with targeted therapy?
Targeted therapy drugs are processed by your liver using specific enzyme pathways. Many common medicines — including some herbal supplements — either speed up or slow down those pathways.
If a medicine speeds them up, your targeted drug is broken down faster and may not reach the level it needs to work. If it slows them down, the drug builds up and the risk of side effects increases.
A few targeted drugs also need a certain level of stomach acid to be absorbed properly. Medicines that routinely reduce acid can limit how much of the drug your body takes in, according to NCCN and ESMO prescribing guidance.
How to review your medicines before your targeted therapy appointment
Write down everything you currently take
Include prescription medicines, over-the-counter tablets, vitamins, mineral supplements, herbal products, and traditional preparations. Include things you take only occasionally, not just daily medicines.
Note the dose and how often you take it
Your team needs this to judge whether the dose or frequency creates a meaningful interaction — not just whether a medicine appears on a list at all.
Bring the written list to your pre-treatment appointment
A written list is more reliable than describing things from memory in clinic. If you have original packaging for supplements, bring that too — ingredient lists are useful.
Tell your oncologist about anything you started recently
A medicine you added in the last few weeks is as relevant as one you have been on for years. Recent changes to dose also matter and should be mentioned.
Wait for specific instructions before stopping anything
Your team will tell you which medicines to stop, which to continue, and how much notice is needed before your first dose. The timeline depends on the medicine and the targeted drug you are starting, so do not guess.
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Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
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MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
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Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Dr. Vinay Mamidala
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Dr. Mohammed Imran
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MBBS, MS (General Surgery), DrNB (Surgical Oncology), FALS Oncology
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What to include on your medicine list — a printable summary
- Every medicine prescribed by any doctor, not only your oncologist
- Over-the-counter painkillers such as ibuprofen, aspirin, or diclofenac
- Heartburn, antacid, and acid-reducing medicines, including those bought without a prescription
- St John's Wort — one of the strongest known reducers of targeted drug levels in the blood
- Any other herbal supplement, whether bought from a pharmacy or a practitioner
- Ayurvedic, Unani, homeopathic, or other traditional preparations, including anything applied to the skin
- Vitamins and mineral supplements, especially those taken in doses above a standard daily amount
- Grapefruit and grapefruit juice — these affect the same liver enzymes as several targeted drugs
- Any medicine you take only occasionally, such as for pain, sleep, anxiety, or allergies
- Medicines from a cardiologist, neurologist, or any other specialist who is not your oncologist
Is it safe to stop a medicine yourself if you think it might clash?
No. Stopping a medicine on your own — even one your team will ultimately ask you to pause — can cause its own harm. Blood pressure medicines, diabetes medicines, antidepressants, and blood thinners all need a careful, supervised plan.
If you see a concern on any list online and feel anxious about it, call your care team rather than acting on it yourself. They can tell you, for your specific targeted drug, which interactions matter and which do not.
The same applies once treatment has started. If a separate doctor prescribes a new medicine during your targeted therapy course, tell both that doctor and your oncology team before you take the first dose.
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Common questions
Frequently asked questions
Will I have to stop my blood pressure or diabetes medicine?
Most people do not have to stop these medicines, but your team needs to review them because a small number of interactions exist between common long-term medicines and specific targeted drugs. Bring your full list and let your oncologist check rather than assuming either way. If a change is needed, your team will plan it carefully with whoever first prescribed that medicine so nothing is stopped abruptly.
Is St John's Wort really a problem — it is just a herbal supplement?
St John's Wort is one of the most clinically significant interactions known for targeted therapy. It activates the same liver enzyme that breaks down many targeted agents and can reduce the level of your drug in the blood to a point where treatment becomes less effective. NCCN and ESMO guidelines consistently list it as a medicine to stop before starting treatment. Being plant-derived does not make it safe to take alongside these drugs.
What about Ayurvedic or homeopathic preparations?
Tell your team about all of them, including anything you apply to the skin. This is not a judgement on those practices — it is because the ingredients in some preparations are not fully labelled, and certain plant compounds affect the same liver pathways as your targeted drug. Your team is not asking you to stop believing in them; they need to know what is in your system so they can make a safe plan.
What if the medicine I need to stop was prescribed by a different doctor?
Your oncologist will communicate with that doctor, or advise you on how to make the change safely. Never stop a medicine prescribed by another specialist on the basis of a list alone — both doctors need to agree on the plan. If your oncologist is uncertain whether an interaction is clinically significant for your specific targeted drug, they can consult the prescribing information or a clinical pharmacist.
How far in advance do I need to stop a medicine before targeted therapy starts?
There is no single answer — the timeline depends on both the medicine you are stopping and the targeted drug you are starting. This is one reason to bring your list to the appointment before your planned start date, so there is time to make any changes that are needed. Your oncologist will give you a specific instruction for each medicine once they have reviewed your full list.