Can hormone therapy be used to prevent breast cancer?

Yes. Most early breast cancers are ER-positive, and clinical trials have studied whether hormone therapy can be used to prevent breast cancer in women who are at increased risk of getting the disease.

A large NCI-sponsored randomized clinical trial called the Breast Cancer Prevention Trial found that tamoxifen, taken for 5 years, reduced the risk of developing invasive breast cancer by about 50 percent in postmenopausal women who were at increased risk of getting the disease (12). A subsequent large randomized trial, the Study of Tamoxifen and Raloxifene, which was also sponsored by NCI, found that 5 years of raloxifene reduces breast cancer risk in such women by about 38 percent (13).

As a result of these trials, both tamoxifen and raloxifene have been approved by the FDA to reduce the risk of developing breast cancer in women at high risk of the disease. Tamoxifen is approved for use regardless of menopausal status. Raloxifene is approved for use only in postmenopausal women.

The aromatase inhibitor exemestane has also been found to reduce the risk of breast cancer in postmenopausal women at increased risk of the disease. After 3 years of follow-up in another randomized trial, women who took exemestane were 65 percent less likely than those who took a placebo to develop breast cancer (14).

Longer follow-up studies will be necessary to determine whether the risk reduction with exemestane remains high over time, as well as to understand any risks of exemestane treatment. Although exemestane has been approved by the FDA for treatment of women with ER-positive breast cancer, it has not been approved for breast cancer prevention.

What are the side effects of hormone therapy?

The side effects of hormone therapy depend largely on the specific drug or the type of treatment (5). The benefits and risks of taking hormone therapy should be carefully weighed for each woman.

Hot flashes, night sweats, and vaginal dryness are common side effects of hormone therapy. Hormone therapy also disrupts the menstrual cycle in premenopausal women.

Less common but serious side effects of hormone therapy drugs are listed below.

Tamoxifen

  • Risk of blood clots, especially in the lungs and legs (12)
  • Stroke (15)
  • Cataracts (16)
  • Endometrial and uterine cancers (15, 17)
  • Bone loss in premenopausal women
  • Mood swings, depression, and loss of libido
  • In men: headaches, nausea, vomiting, skin rash, impotence, and decreased sexual interest

Raloxifene

  • Risk of blood clots, especially in the lungs and legs (12)
  • Stroke in certain subgroups (15)

Ovarian suppression

  • Bone loss
  • Mood swings, depression, and loss of libido

Aromatase inhibitors

  • Risk of heart attack, angina, heart failure, and hypercholesterolemia (18)
  • Bone loss
  • Joint pain (19–22)
  • Mood swings and depression

Fulvestrant

  • Gastrointestinal symptoms (23)
  • Loss of strength (23)
  • Pain

A common switching strategy, in which patients take tamoxifen for 2 or 3 years, followed by an aromatase inhibitor for 2 or 3 years, may yield the best balance of benefits and harms of these two types of hormone therapy (15).