Tamoxifen is a medicine used against breast cancer. Many breast cancers grow because the body's own hormone, oestrogen, feeds them. Tamoxifen sits in the place where the hormone would attach to the cancer cell, so the hormone cannot get in. It does not kill the cancer directly; it takes away what the cancer was living on.

Patients usually take it as a daily tablet for several years after surgery, to stop the cancer coming back, and doctors sometimes give it to people at high risk who do not have cancer yet. Common side effects are hot flushes and feeling tired — many patients say the medicine is not hard to take but is hard to keep taking for five or ten years.

The word itself is a drug name, so it stays the same everywhere and takes no capital letter (Nolvadex, with a capital, is one brand version of it). You will meet it in health news and in doctors' talk, not in everyday conversation. If a student sees it, it is almost always in the phrase "on tamoxifen" — meaning taking it as ongoing treatment.