Oncology
Tamoxifen
A selective oestrogen receptor modulator (SERM) — A long-term hormone treatment that reduces the risk of breast-cancer recurrence.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Tamoxifen is a long-term endocrine (hormone) treatment for breast cancers that are driven by oestrogen, used after surgery to prevent recurrence and sometimes to reduce risk in women at high risk. It is also used in some pre-menopausal settings where aromatase inhibitors are unsuitable.
How it works
Many breast cancers are stimulated by oestrogen. Tamoxifen blocks the oestrogen receptor in breast tissue, depriving the cancer of that growth signal. In other tissues, such as the womb lining and bones, it behaves more like oestrogen — which explains both some of its side-effects and some of its other effects.
In practice
In practice tamoxifen is a cornerstone of treatment for hormone-receptor-positive breast cancer, taken for several years to substantially cut the risk of recurrence, and it is also used for risk reduction in some women at high risk. Because it acts on oestrogen receptors differently in different tissues, its key risks reflect oestrogen-like effects in some places: an increased risk of blood clots and of endometrial (womb-lining) changes including cancer, so any abnormal vaginal bleeding must be reported and investigated. Hot flushes and menopausal-type symptoms are common and a frequent reason for poor adherence, so they are actively managed. It interacts with drugs that block the enzyme (CYP2D6) needed to activate it — notably some antidepressants used for hot flushes — which can reduce its effectiveness, so those combinations are chosen carefully. It is stopped before pregnancy and is teratogenic.
Examples
Practical use
How to take it & use it well
- Tamoxifen is taken by mouth, usually once a day, often for several years to treat or reduce the risk of certain breast cancers.
- Take it at around the same time each day; you can take it with or without food, and swallow tablets with water.
- Keep taking it even if you feel well, as it works over the long term; do not stop without discussing it with your specialist.
- Tell your team about any unusual vaginal bleeding, leg swelling or pain, or sudden breathlessness, as these need urgent assessment.
- Effective non-hormonal contraception is needed during treatment and for a time afterwards because tamoxifen can harm a developing baby.
- If you miss a dose, take it when you remember unless it is nearly time for the next one; do not double up.
Common uses
- Hormone-receptor-positive breast cancer (prevention of recurrence)
- Risk reduction in some high-risk women
- Selected pre-menopausal breast-cancer settings
Monitoring
- Adherence and management of menopausal-type side-effects
- Any abnormal vaginal bleeding (needs assessment)
- Interacting medicines (CYP2D6 inhibitors) and clot risk
Weighing it up
Advantages & disadvantages
Advantages
- Reduces the risk of breast cancer returning and improves survival in hormone-sensitive disease.
- Can be used in women before and after the menopause.
- Taken as a simple once-daily tablet at home.
- Also used to lower breast cancer risk in some people at high risk.
Disadvantages
- Increases the risk of blood clots in the legs and lungs.
- Raises the risk of changes to the womb lining, including, rarely, womb cancer, so new bleeding must be reported.
- Commonly causes hot flushes, sweats, mood changes and menstrual changes.
- Can cause harm to a developing baby, so pregnancy must be avoided.
- May rarely affect the eyes, so report any vision changes.
Key safety principles
What to watch for
- Increased risk of blood clots and of endometrial changes/cancer — report any abnormal vaginal bleeding.
- Interacts with CYP2D6-blocking drugs (e.g. some antidepressants) which can reduce its effectiveness.
- Hot flushes and menopausal symptoms are common; it is teratogenic — avoid in pregnancy and stop beforehand.
Key interactions
What to avoid or check alongside
- Strong CYP2D6-inhibiting antidepressants such as paroxetine and fluoxetine may reduce tamoxifen's activation and are usually avoided; alternatives are chosen where possible.
- Warfarin and other anticoagulants can have their blood-thinning effect increased, raising bleeding risk and needing closer monitoring.
- Combining with other medicines that raise clot risk, such as some hormone treatments, adds to the danger of thrombosis.
- Certain medicines that affect liver enzymes can change tamoxifen levels, so all medicines and supplements should be reviewed.
Patient & carer advice
- Taking it consistently for the full course gives the protection — tell us if side-effects make that hard
- Report any unusual vaginal bleeding promptly
- Check with us before starting other medicines, and use reliable non-hormonal contraception
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Tamoxifen: frequently asked questions
Why do I have to take tamoxifen for several years?
Tamoxifen works by blocking the effect of oestrogen on breast cancer cells over the long term. Taking it for the full course your specialist advises gives the best protection against the cancer returning.
Can I take tamoxifen if I might become pregnant?
No. Tamoxifen can seriously harm a developing baby, so reliable non-hormonal contraception is needed during treatment and for a period after stopping. Tell your team straight away if you could be pregnant.
Which antidepressants should I avoid with tamoxifen?
Some antidepressants, especially paroxetine and fluoxetine, can reduce how well tamoxifen works by blocking the enzyme that activates it. Always tell your prescriber you take tamoxifen so a safer option can be chosen if needed.
Is vaginal bleeding on tamoxifen serious?
Any new or unusual vaginal bleeding should be reported promptly, as tamoxifen can cause changes to the womb lining that need checking, including rare cases of womb cancer.
Will tamoxifen cause hot flushes?
Hot flushes and sweats are common with tamoxifen and often settle with time. If they are troubling you, your team can suggest ways to manage them.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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