Which supplements interact with breast cancer medicines?

Many people take vitamins, minerals, herbal products or traditional remedies during and after breast cancer treatment, often hoping to boost immunity, ease side effects or support recovery. While some supplements are harmless or helpful, others can interact with cancer medicines, making them less effective, increasing side effects, or affecting blood tests and surgery. The liver processes many cancer medicines using enzymes, and some herbs speed up or slow down these enzymes. For example, St John's wort can lower levels of tamoxifen, aromatase inhibitors such as letrozole, CDK4/6 inhibitors such as palbociclib, ribociclib and abemaciclib, and some chemotherapy medicines, potentially reducing their effect. Grapefruit and its juice can raise levels of certain targeted medicines, increasing side effects. Tamoxifen needs to be converted into an active form in the liver, and some supplements and medicines may interfere with this. High-dose antioxidants such as vitamin C, vitamin E or selenium may, in theory, reduce the effect of some chemotherapy and radiation. Supplements with plant oestrogens, such as concentrated soy isoflavones, red clover and some menopause products, may counteract hormone therapy. Turmeric supplements, ginkgo, garlic, fish oil and vitamin E in high doses can increase bleeding risk, which matters around surgery or when platelets are low. Always tell your oncologist and pharmacist about every supplement before starting or stopping it.

Natural does not mean safe

Herbs and vitamins can have strong effects on how medicines work.

Timing matters

Some supplements are more concerning during chemotherapy, radiation or surgery.

Share everything

Tell your team about all products, including Ayurvedic and homeopathic remedies.

Understanding supplement interactions involves some uncertainty.

Evidence is often limited

Many interactions are known from laboratory studies or case reports rather than large trials.

Products vary

Supplement strength and purity differ between brands and batches.

Some supplements are helpful

Vitamin D and calcium are often recommended for bone health.

Doctors may not ask

You may need to raise supplements yourself at appointments.

What this page cannot tell you

It cannot list every interaction. Your pharmacist can check your products.

Many families use traditional remedies alongside modern medicine.

Share details

Tell your team the name, ingredients and dose of each product.

Quality concerns

Some traditional preparations have been found to contain heavy metals or undeclared medicines.

Liver effects

Some herbal mixtures can affect the liver, which processes many cancer medicines.

Do not replace treatment

Traditional remedies should never replace recommended cancer treatment.

Tamoxifen is a prodrug, which means it is converted in the liver into active forms, mainly endoxifen, that do most of its work against breast cancer. This conversion depends heavily on a liver enzyme called CYP2D6.

Medicines that block the enzyme

Some antidepressants, such as paroxetine and fluoxetine, strongly block CYP2D6 and may lower endoxifen levels. Doctors usually prefer alternatives that interact less when an antidepressant is needed during tamoxifen treatment.

Herbal products

St John's wort speeds up other liver enzymes and may change tamoxifen levels. Some other herbal products have been suggested to affect these enzymes, though evidence is limited.

Hormone-like supplements

Products containing plant oestrogens may, in theory, compete with tamoxifen in breast tissue.

Blood clot risk

Tamoxifen slightly increases the risk of blood clots. Supplements promoted for circulation or weight loss should be checked for effects on clotting.

What you can do

Keep a complete list of medicines and supplements, and ask your pharmacist to check for interactions whenever anything changes.