Tamoxifen for Breast Cancer: Uses, Side Effects & How Long You Take It

No. Tamoxifen is hormone (endocrine) therapy, not chemotherapy. It works by blocking oestrogen from feeding hormone-receptor-positive breast cancer cells, rather than by destroying fast-dividing cells the way chemotherapy does. That is why it is taken as a daily tablet at home and does not cause the hair loss or low blood counts typical of chemo. Some women have chemotherapy first and then take tamoxifen afterwards to lower the chance of the cancer returning. Your oncologist will explain exactly where tamoxifen fits in your overall treatment plan.

It depends on your individual risk of recurrence. Five years is the proven minimum for early hormone-positive breast cancer. The ATLAS trial showed that continuing to 10 years further reduced both recurrence and deaths from breast cancer, with the benefit growing in the years after treatment. For premenopausal women, guidelines support starting with 5 years and continuing to 10 if you remain premenopausal and are tolerating it well. At CION this decision is reviewed by our tumor board, weighing your recurrence risk against side effects, so you get a team recommendation rather than a one-size answer.

Although weight gain is sometimes listed as a side effect, good research does not show a strong link between tamoxifen itself and gaining weight. One study of over 3,000 breast cancer survivors found tamoxifen was not clearly associated with weight gain, while chemotherapy was. Weight often changes during treatment because of reduced activity, menopause or other medicines rather than the tamoxifen tablet. Regular walking, a balanced diet rich in vegetables and lean protein, and support from our nutritionist all help. If you are gaining weight and it worries you, tell your oncologist so other causes can be checked.

Tamoxifen can thicken the lining of the womb and slightly increases the risk of endometrial cancer, but this risk mainly applies to postmenopausal women; premenopausal women are not affected in the same way. The overall risk is small and is far outweighed by tamoxifen's benefit in preventing breast cancer recurrence. The key safety step is simple: report any abnormal vaginal bleeding to your doctor promptly so it can be investigated early. At CION we arrange a gynaecology review for any unexpected bleeding, because problems found early are the most treatable.

Tell your oncologist about every medicine and supplement you take, because some interact with tamoxifen. Certain antidepressants (fluoxetine, paroxetine, bupropion) can make it less effective, so these should be reviewed rather than stopped on your own. Avoid taking tamoxifen at the same time as aromatase inhibitors. Grapefruit and grapefruit juice may interfere with how the drug is processed, and heavy alcohol can worsen hot flushes. Avoid oestrogen-containing products and discuss any herbal remedy before using it. Do not get pregnant while taking tamoxifen, as it can cause birth defects.

No. Tamoxifen can cause birth defects, so you must not be pregnant or breastfeed while taking it. Because it does not act as a contraceptive and you can still conceive, use non-hormonal contraception (such as condoms or a copper IUD) during treatment and for about two months after stopping. If you hope to have children, raise it before you start so fertility preservation can be arranged. The recent POSITIVE trial showed that some women can safely pause hormone therapy for a planned time to try for a pregnancy and then resume — a decision made carefully with your oncologist.

Common side effects such as hot flushes, night sweats or mild nausea often begin within the first few weeks and frequently ease over the first few months as your body adjusts. Some effects, like changes to periods, may persist while you are on treatment. Serious effects such as blood clots or abnormal womb bleeding are uncommon but need prompt attention at any point. The most important thing is not to suffer in silence: most side effects can be reduced through simple measures, dose timing or supportive care. Tell your CION oncologist about anything new at your follow-up visits.

Yes — tamoxifen is the main hormone therapy for premenopausal women with hormone-receptor-positive breast cancer. This is because aromatase inhibitors do not work on their own before menopause. Depending on your risk, your oncologist may combine tamoxifen with ovarian suppression for added protection. Premenopausal women also do not carry the same endometrial cancer risk that postmenopausal women do. Because younger women face issues around periods, fertility and pregnancy, we discuss all of this openly at your first 45-minute consultation so your plan fits your life as well as your cancer.