Medicines and supplements during cancer treatment: interactions to know about

Cancer drugs have narrow margins: too little may not work, too much can harm. Many are broken down by the same liver enzymes that process herbs and other medicines, and some work by damaging fast-dividing cells, which antioxidants could in theory protect. That is why the advice from Cancer Research UK is blunt: some therapies may be harmful or interact with the treatment you are having, so talk to your doctor before starting.

This is not a criticism of wanting to take action. Many people feel better doing something, and the team can help you find options that are safe, such as nutrition advice, gentle exercise and symptom support.

Cancer Research UK says there is no reliable evidence that dietary or nutritional supplements can prevent, cure or control cancer. It says high-dose antioxidant supplements such as vitamins A, C and E, selenium and coenzyme Q10 may reduce how well treatment works, and that high-dose products can themselves cause toxicity.

Individual examples on its page include vitamin C, for which there is no quality evidence of improved chemotherapy outcomes, and selenium, where some trials showed a higher risk of high-grade prostate cancer. Vitamin D is treated differently because it supports bone health; the page cites the UK Government advice for the general population and for people at higher risk, and the team can say whether a supplement suits you.

Cancer Research UK says there is not enough reliable scientific evidence to use herbal medicine as a cancer treatment. Some studies of Chinese herbal medicine hint at help with symptoms and quality of life, but no human trial has shown that herbs treat or cure cancer.

It also warns about safety. In the UK, a Traditional Herbal Registration mark shows a product has been assessed for quality and safety, whereas some unlicensed herbal products have been found to contain illegal substances or toxic herbs not listed on the label. Asian ginseng and bilberry can raise bleeding risk, which matters around surgery or when platelets are low.

St John's wort is the best-documented example of a problem product. Cancer Research UK says it can change how well some drugs work, making them stronger or weaker, and names irinotecan, dasatinib, erlotinib, imatinib, sorafenib, sunitinib, etoposide and mitotane. It may also interact with blood thinners, antibiotics, heart medicines, HIV drugs, contraceptives and other antidepressants, and can make skin more sensitive during radiotherapy.

Turmeric and curcumin illustrate a different problem: laboratory promise without clear human evidence. Cancer Research UK says there is no clear evidence that they prevent or treat cancer, that curcumin is poorly absorbed, and that it might interact with other medicines. It also warns that some products sold under certain brand names have contained nimesulide, which can cause severe liver damage.

The NHS lists common chemotherapy effects such as tiredness, sickness, mouth soreness, a weakened immune system, easy bruising or bleeding, and numbness or tingling. It says to contact your care team immediately or call NHS 111 for a high temperature, sore throat, cough or urinary symptoms, painful swelling in an arm or leg, or sudden breathlessness or chest pain.

Over-the-counter medicines deserve the same care as supplements. Painkillers that thin the blood or irritate the stomach, remedies for heartburn and laxatives can each matter, and a fever treated at home before ringing the team can hide an infection. The NHS page advises discussing any medicine or supplement with the care team first.

Before starting a supplement, take the label, or a photo of it, to your oncology pharmacist or specialist nurse. Ask what it is expected to do, whether it affects your specific drugs, and whether it should be paused around treatment days or surgery. Ask about products from friends and relatives too.

If you are tempted to stop a treatment for an alternative, talk it through first. Cancer Research UK warns that you could harm your health if you stop cancer treatment for an unproven one.