Tamoxifen is a drug that can block the action of oestrogen, the female hormone, on breast cancer cells. It is an unusual drug because although it has an anti-oestrogen effect on breast cells it has a slight weak oestrogen action on other organs such as the womb, vagina, bone and heart. These effects explain for the most part the side effects that this drug can cause.
The majority of breast cancers respond to tamoxifen. Sometimes oestrogen receptor levels in breast cancer tissue are used to indicate whether the cancer will respond (see section on Interpreting the Biopsy Report). It is more likely the breast cancer will respond if it is oestrogen receptor positive but we also know up to 20 percent of oestrogen receptor negative cancers will respond. Because of this most postmenopausal women will go on tamoxifen regardless of their breast cancer oestrogen receptor status.
We use tamoxifen more selectively in premenopausal patients and generally give it to only women who have oestrogen receptor positive cancers. As a rule, the best results are seen in postmenopausal patients.
In premenopausal women it can cause:
- Irregular vaginal bleeding
- Flushes – especially if women are approaching menopause.
- Ovarian cysts – Tamoxifen can directly affect the ovary. Please report any pelvic pain to your doctor.Women are advised not to get pregnant while on this drug and unless one of the partners has been sterilised, a barrier method of contraception such as condoms is indicated as the action of the oral contraceptive pill may be affected.
In postmenopausal women it can cause:
- Hot flushes – may be exacerbated especially for women who have recently gone through the menopause.
- Slight vaginal discharge – women who have been postmenopausal for a reasonable length of time may notice this. This is quite normal and simply due to the weak oestrogen effects on the bone and heart similar to hormone replacement therapy which protects against heart disease and bone loss.Post menopausal women on tamoxifen should not get any vaginal bleeding on tamoxifen so if this occurs it should be reported to your doctor. There have been a few, rare cases of uterine cancer reported in postmenopausal women on long-term tamoxifen.
In both pre and postmenopausal women:
Nausea may be a problem which can be avoided by either taking the drug at night, with a meal, or by taking tamoxifen in a twice a day dose instead of a single dose.
Vaginal irritation can be a troublesome side effect that may warrant stopping the drug. It is not due to vaginal ‘thrush’ and usual anti-thrush medications do not help long-term as this side-effect is recurring.
Skin itching may occur a few weeks after starting the drug. Anti-itching medications will help control the symptoms. Sometimes the only way to cure this problem is by stopping the drug.
Premenstrual-like moodiness/depression may be experienced on this drug. It is not common but this side effect would probably also warrant stopping tamoxifen.
Blurred vision is an uncommon side effect but if this does occur an eye check by an optician or eye specialist is recommended.
Fluid retention may be exacerbated. Diuretic therapy for conditions such as congestive heart failure may need to be adjusted accordingly .
Blood clotting in the veins is a rare complication. Report any swelling, pain or tightness in the legs or extremities.
Weight gain (small) may be noticed by women who have a tendency for this, so …….. fore-warned is fore-armed! It is not impossible to lose weight on tamoxifen .
Women who are taking tamoxifen for the treatment of breast cancer need to be on it for at least two years and possibly up to five years. It is unclear how long this drug should be continued – international trials are nearing completion at the present time so it is hoped that the answer to this question will soon be available.
Tamoxifen is a remarkably well tolerated drug with very few side effects. The vast majority of women taking this drug have no problems at all.