Tamoxifen Side Effects

Tamoxifen, the oldest endocrine or hormonal therapy used to treat ER-positive breast cancer, has been widely used since 1998. It is used to treat early stage breast cancers in men and women, sometimes after chemotherapy and sometimes as a stand-alone treatment. It is also used to treat metastatic or advanced breast cancer or as chemoprevention to prevent breast cancer developing in women at high risk.

Given its positive benefits, why are some women worried about taking Tamoxifen?

Generally, Tamoxifen is prescribed for younger/premenopausal women, although for many years it was the only available endocrine therapy for women of any age. More recently, postmenopausal women usually are given one of the aromatase inhibitors (AIs).

An extremely powerful drug, Tamoxifen, or Tam as it is sometimes called, can reduce the risk of a breast cancer recurrence by 30% to 50% in premenopausal women. That is equivalent to the usual benefit of chemotherapy. It can also reduce the risk of developing another breast cancer in the other breast by about 50%. Sometimes it is used to try to shrink large breast tumors before surgery, making it possible for a woman to have a lumpectomy/wide excision rather than a mastectomy.

Additionally, it can offer other benefits that aren't directly related to cancer treatment. While it blocks estrogen's activity in cancer cells, it actually increases the benefit of estrogen in bone and liver cells. This means that it may help sustain healthy bones and healthy cholesterol levels after menopause.

Given its positive benefits, why are some women worried about taking Tamoxifen?

There are a few potentially serious, but quite rare, side effects of the drug: endometrial cancer and blood clots. Remind yourself of the rarity of both of these and that both would announce themselves with obvious symptoms. Through four decades, I have known only one woman who developed endometrial cancer on Tamoxifen; she experienced vaginal bleeding, was quickly diagnosed and cured by surgery. I have also only known one woman who developed a blood clot. She had intense pain in her leg, went to the ER, was diagnosed and put on blood-thinners that fixed the problem. Tamoxifen can also increase the risk of a stroke, especially in people with pre-existing cardiovascular issues. These are reasons that your doctor will take a careful history before prescribing the medication.

There are other more common possible side effects.

These include bone pain similar to arthritis, hot flashes, fatigue, hair thinning, mood swings, and diminished libido.

Most women describe the bone pain not as pain but as stiffness or achiness. It is usually worst first thing in the morning or getting out of the car after a long drive. One can feel 100 years old for the first few stiff steps, and then it rapidly improves.

The hot flashes are annoying indeed and highly variable. Some women have none, some have warm flashes, and some are awakened at night in a hot sweat. It is impossible to predict how long they might persist, but everyone has reduced incidence of hot flashes eventually.

Fatigue is hard to quantify as it so easily can be related to other factors, but it remains on the list.

The possible hair thinning is related to loss of estrogen and is nothing like hair loss from chemo. No one goes bald from Tamoxifen, but some people do experience thinning hair.

Like hair thinning, a lowered libido is common postmenopause and in the setting of reduce estrogen.

Some women also attribute weight gain to Tamoxifen. As I have listened to women over the years describe this situation, it seems more accurate to describe it as being more difficult to lose weight or to maintain. We are not talking a 30-pound weight gain here, more like 5 to 10 pounds. Again, this frequently happens to women postmenopause and usually accompanies the loss of the wasp waist we all enjoyed in our youth.

How long do women take Tamoxifen?

The answer is highly variable, and the recommendations seem to change over time. The American Society of Clinical Oncology (ASCO) recommends that newly diagnosed premenopausal and perimenopausal women take the drug for 5 years. After this, some women stop, some continue with Tam, and some switch to one of the aromatase inhibitors for up to another 5 years. Postmenopausal women are sometimes first prescribed Tamoxifen, but, more often, begin treatment with one of the AIs for up to 10 years. Again, it is tricky to put any numbers here as everyone's situation is different, the recommendations change, and this decision will be up to you and your doctor.

My own experience, as well as that of most women whom I have known, is that taking Tamoxifen is not a hardship. We are all grateful for its power in reducing the risk of a cancer recurrence and most of us do not experience any difficult side effects. For most, the hardest part is just remembering to take the small pill daily.