What should you know about flaxseed and tamoxifen?

Tamoxifen is changed by the body into active forms that help block oestrogen signals in some breast cancers. Flaxseed contains fibre and plant compounds called lignans. Food amounts may not act like a concentrated powder, oil, capsule or extract, and products can vary in strength. Fibre can also change how some medicines are absorbed if taken at the same time. These differences make a personalised check more useful than treating every flaxseed product as equivalent.

Tell your prescriber or oncology pharmacist the form and amount of flaxseed you use, including smoothies, supplements and products bought online. Do not stop tamoxifen or replace it with flaxseed. If your team permits flaxseed, follow their advice about timing and preparation, and keep the product consistent so changes are easier to recognise. Bring the supplement packaging to an appointment.

Contact your cancer team promptly if you develop unexpected vaginal bleeding, severe headache, chest pain, sudden breathlessness, leg swelling or leg pain while taking tamoxifen. Seek urgent help for severe breathing difficulty, fainting or sudden weakness. These symptoms can have causes unrelated to flaxseed, but they should not be assessed by changing supplements alone.

Ask whether your usual culinary amount is acceptable, whether a supplement or oil should be avoided, and whether you should separate flaxseed from tamoxifen or other medicines. Check who should review any future herbal product before you start it.