Tamoxifen: what it does and what to watch

Also known as: Selective estrogen receptor modulator, SERM · Hormone therapy (selective estrogen receptor modulator) · Updated 2026-10-25 · Educational — not medical advice

Illustrative — a cell and its nucleus, where hormone signals are received

In plain language: This medicine blocks estrogen from attaching to its receptor in breast tissue, slowing or stopping the growth of cancer cells that depend on estrogen. In other tissues, such as the uterus, it can behave more like estrogen itself, which explains why it has different, and sometimes opposite, effects in different parts of the body.

What it's used for

  • Treatment of hormone-receptor-positive breast cancer
  • Reducing the risk of breast cancer recurrence after primary treatment
  • Reducing breast cancer risk in some people at high risk

Common side effects

  • Hot flushes
  • Night sweats
  • Vaginal dryness or discharge
  • Mood changes
  • Fatigue
  • Irregular menstrual periods

Get medical help if…

Increased risk of blood clots in the legs or lungs

Changes in the uterine lining, including a small increased risk of endometrial cancer

Rare vision changes affecting the retina or cornea with long-term use

Liver changes

Stroke: sudden weakness, numbness or tingling in the face, arm or leg, especially on one side, sudden trouble speaking or seeing, or a sudden severe headache. Call emergency services now.

Lab values it can move

Few guides tell you this part: medicines show up in blood work. If you take Tamoxifen and a value below looks off on your report, this may be part of the story — never stop a medicine without your doctor.

Liver enzymes may be checked periodically, since this medicine is processed by the liver and can occasionally affect it.

SGPT (ALT)

This medicine can raise triglyceride levels in some people, so blood fat levels may be monitored, particularly with existing high triglycerides.

Triglycerides

Good to know

  • Report any unusual vaginal bleeding or spotting promptly, since this needs evaluation of the uterine lining
  • Certain antidepressant medicines can reduce how well this medicine works and should be discussed with a prescriber before starting either
  • Sudden breathlessness or chest pain: call emergency services now, as these can signal a blood clot in the lungs. Pain, swelling, warmth or redness in one calf or leg (possible blood clot): see a doctor today
  • Regular gynaecological and eye check-ups may be recommended with long-term use
  • This medicine is usually taken for several years exactly as advised by an oncology team
  • Can harm an unborn baby. Avoid pregnancy during treatment and for 2 months after it, using a reliable non-hormonal method of contraception, and do not breastfeed during treatment. Stop taking it and call your doctor right away if you think you are pregnant.
  • Tell your doctor if you take warfarin or another blood thinner. Tamoxifen can strongly increase its effect, so clotting tests are watched closely, and the combination is avoided in some uses of tamoxifen.

Questions people ask

Can I take any antidepressant alongside this medicine?

Not necessarily. Some antidepressants interfere with an enzyme needed to convert this medicine into its most active form, which can reduce how well it protects against cancer recurrence. If an antidepressant is needed for mood symptoms, it is important to discuss options with a prescriber so one with less interaction potential can be chosen where possible.

Why should I report any unusual vaginal bleeding?

This medicine can act like estrogen on the lining of the uterus, causing thickening that in rare cases is linked to endometrial cancer. Unusual bleeding or spotting, especially after menopause, is the main warning sign and needs prompt evaluation, often with an ultrasound or biopsy, so any changes should be reported without delay.

Sources

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