What are the drugs tamoxifen and toremifene?

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Tamoxifen and toremifene (Fareston) are anti-estrogen drugs that work by temporarily blocking estrogen receptors on breast cancer cells, preventing estrogen from binding to them. They are taken daily as a pill.
For women with ER- or PR-positive cancers, taking tamoxifen after surgery for 5 years reduces the chances of the cancer coming back by about half. Tamoxifen can also be used to treat metastatic breast cancer, as well as to reduce the risk of developing breast cancer in women at high risk. Toremifene works like tamoxifen, but is not used as often.
The most common side effects of these drugs include fatigue, hot flashes, vaginal dryness or discharge, and mood swings.
Some patients whose cancer has spread to their bones may experience a "tumor flare" with pain and swelling in the muscles and bones. This usually subsides quickly, but in some cases the patient may also develop a high calcium level in the blood that cannot be controlled. If this occurs, the treatment may need to be stopped.
Rare, but more serious side effects are also possible. These drugs can increase the risk of developing cancers of the uterus (endometrial cancer and uterine sarcoma). Tell your doctor right away about any unusual vaginal bleeding (a common symptom of both of these cancers). Most uterine bleeding is not from cancer, but this symptom always needs prompt attention.
Another possible serious side effect is blood clots, which usually form in the legs. In some cases, these may lead to a heart attack, stroke, or blockage in the lungs (pulmonary embolism). Call your doctor or nurse right away if you develop pain, redness, or swelling in your lower leg (calf), shortness of breath, chest pain, sudden severe headache, confusion, or trouble speaking or moving.
Depending on a woman's menopausal status, tamoxifen can have different effects on the bones. In pre-menopausal women tamoxifen can cause some bone thinning, but in post-menopausal women it is often good for bone strength. The effects of toremifene on the bones are less clear.

For most women with breast cancer, the benefits of taking these drugs outweigh the risks.