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Am Fam Physician. 2003;67(6):1309-1314

A more recent USPSTF on this topic is available.

This statement summarizes the current U.S. Preventive Services Task Force (USPSTF) recommendations for the chemoprevention of breast cancer and the supporting scientific evidence. Explanations of the current ratings and of the strength of overall evidence are given in Tables 1 and 2, respectively. The complete information on which this statement is based is available in the article “Chemo-prevention of Breast Cancer: A Summary of the Evidence”1 and in the summary of the evidence and Systematic Evidence Review2 on this topic, available through the USPSTF Web site ( and the National Guideline Clearinghouse ( A summary of the evidence is also available in print through the Agency for Healthcare Research and Quality Publications Clearinghouse (telephone: 800-358-9295;

The USPSTF grades its recommendations according to one of five classifications (A, B, C, D, or I) reflecting the strength of evidence and magnitude of net benefit (benefits minus harms).
A.The USPSTF strongly recommends that clinicians provide [the service] to eligible patients. The USPSTF found good evidence that [the service] improves important health outcomes and concludes that benefits substantially outweigh harms.
B.The USPSTF recommends that clinicians provide [the service] to eligible patients. The USPSTF found at least fair evidence that [the service] improves important health outcomes and concludes that benefits outweigh harms.
C.The USPSTF makes no recommendation for or against routine provision of [the service]. The USPSTF found at least fair evidence that [the service] can improve health outcomes but concludes that the balance of benefits and harms is too close to justify a general recommendation.
D.The USPSTF recommends against routinely providing [the service] to asymptomatic patients. The USPSTF found at least fair evidence that [the service] is ineffective or that harms outweigh benefits.
I.The USPSTF concludes that the evidence is insufficient to recommend for or against routinely providing [the service]. Evidence that [the service] is effective is lacking, of poor quality, or conflicting, and the balance of benefits and harms cannot be determined.
The USPSTF grades the quality of the overall evidence for a service on a three-point scale (good, fair, or poor).
Good:Evidence includes consistent results from well-designed, well-conducted studies in representative populations that directly assess effects on health outcomes.
Fair:Evidence is sufficient to determine effects on health outcomes, but the strength of the evidence is limited by the number, quality, or consistency of the individual studies; generalizability to routine practice; or indirect nature of the evidence on health outcomes.
Poor:Evidence is insufficient to assess the effects on health outcomes because of limited number or power of studies, important flaws in their design or conduct, gaps in the chain of evidence, or lack of information on important health outcomes.

Summary of Recommendations

  • The USPSTF recommends against the routine use of tamoxifen or raloxifene for the primary prevention of breast cancer in women at low or average risk for breast cancer (See “Clinical Considerations” for a discussion of risk).(D recommendation)

The USPSTF found fair evidence that tamoxifen and raloxifene may prevent some breast cancers in women at low or average risk, based on extrapolation from studies of women at higher risk. The USPSTF concluded, however, that the potential harms of chemoprevention may outweigh the potential benefits in women who are not at high risk for breast cancer.

  • The USPSTF recommends that clinicians discuss chemoprevention with women at high risk for breast cancer and at low risk for adverse effects of chemoprevention. Clinicians should inform patients of the potential benefits and harms of chemoprevention.(B recommendation)

The USPSTF found fair evidence that treatment with tamoxifen can significantly reduce the risk for invasive estrogen-receptor-positive breast cancer in women at high risk for breast cancer and that the likelihood of benefit increases as the risk for breast cancer increases. The USPSTF found consistent but less abundant evidence for the benefit of raloxifene. The USPSTF found good evidence that tamoxifen and raloxifene increase the risk for thromboembolic events (for example, stroke, pulmonary embolism, and deep venous thrombosis) and symptomatic side effects (for example, hot flashes) and that tamoxifen, but not raloxifene, increases the risk for endometrial cancer. The USPSTF concluded that the balance of benefits and harms may be favorable for some high-risk women but will depend on breast cancer risk, risk for potential harms, and individual patient preferences.

Clinical Considerations

  • Clinicians should consider both the risk for breast cancer and the risk for adverse effects when identifying women who may be candidates for chemoprevention.

Risk for breast cancer

Older age; a family history of breast cancer in a mother, sister, or daughter; and a history of atypical hyperplasia on a breast biopsy are the strongest risk factors for breast cancer. Table 33 indicates how the estimated benefits of tamoxifen vary depending on age and family history. Other factors that contribute to risk include race, early age at menarche, pregnancy history (nulliparity or older age at first birth), and number of breast biopsies. The risk for developing breast cancer within five years can be estimated using risk-factor information by completing the National Cancer Institute Breast Cancer Risk Tool (the Gail model, available or 800-4-CANCER [800-422-6237]). Clinicians can use this information to help patients considering tamoxifen therapy estimate the potential benefit. However, the validity, feasibility, and impact of using the Gail model to identify appropriate candidates for chemoprevention has not been tested in a primary care setting. The Gail model does not incorporate estradiol levels or estrogen use, factors that some studies suggest may influence the effectiveness of tamoxifen.

VariableWomen 45 years of ageWomen 55 years of ageWomen 65 years of ageWomen 75 years of age
Percent of predicted five-year risk of breast cancer‡
No family history0.7 percent1.1 percent1.5 percent1.6 percent
Family history1.6 percent2.3 percent3.2 percent3.4 percent
Benefits per 1,000 women over five years of tamoxifen therapy
Number of cases of invasive breast cancer avoided
No family history3 to 45 to 67 to 88
Family history811 to 121617
Cases of noninvasive breast cancer avoided
No family1 to 222 to 32 to 3
Family history2 to 33 to 44 to 55 to 6
Number of hip fractures avoided§<13515
Number of harms per 1,000 women over five years of tamoxifen therapy
Cases of endometrial cancer caused§1 to 2122122
Strokes caused§13920
Pulmonary emboli caused§1 to 24 to 5918
Cases of deep venous thrombosis caused§1 to 21 to 234

Risk for adverse effects

Women are at lower risk for adverse effects from chemoprevention if they are younger; have no predisposition to thromboembolic events such as stroke, pulmonary embolism, or deep venous thrombosis; or do not have a uterus.

  • In general, the balance of benefits and harms of chemoprevention is more favorable for women in their 40s who are at increased risk for breast cancer and have no predisposition to thromboembolic events and women in their 50s who are at increased risk for breast cancer, have no predisposition to thromboembolic events, and do not have a uterus. For example, a woman who is 45 years of age and has a mother, sister, or daughter with breast cancer would have approximately a 1.6 percent risk for developing breast cancer over five years. On average, treating such women with tamoxifen for five years would prevent about three times as many invasive cancers (eight per 1,000) as the number of serious thromboembolic complications caused (one stroke and one to two pulmonary emboli per 1,000). Among women 55 years of age, benefits exceed harms only for those who are not at risk for endometrial cancer, and the margin of benefit is small unless risk for breast cancer is substantially increased (for example, 4 percent over five years).

  • Women younger than 40 years of age have a lower risk for breast cancer and thus will not experience as large an absolute benefit from breast cancer chemoprevention as older women. Women 60 years of age and older, who have the highest risk for breast cancer, also have the highest risk for complications from chemoprevention with a less favorable balance of benefits and harms.

  • The USPSTF found more evidence for the benefits of tamoxifen than for the benefits of raloxifene. Currently, only tamoxifen is approved by the U.S. Food and Drug Administration (FDA) for the specific indication of breast cancer chemoprevention. Although there are biological reasons to suspect that raloxifene should have similar benefits, trial data currently are limited to one study in which the primary outcome was fracture prevention. Additional trials to further evaluate this drug's efficacy for breast cancer chemoprevention are underway, including a trial comparing efficacy and safety of raloxifene and tamoxifen. Raloxifene is approved by the FDA for preventing and treating osteoporosis.

Scientific Evidence


Breast cancer is the most common non-skin cancer in women. An estimated 203,500 new cases of invasive breast cancer will be diagnosed in 2002, and 39,600 women will die from the disease.4 Although the USPSTF concluded that early detection of breast cancer through mammography has reduced deaths from breast cancer, the effectiveness of mammography is limited. Another approach to reducing breast cancer deaths is chemoprevention for primary prevention of cancer.


The use of agents to prevent the development of breast cancer was suggested by trials of breast cancer treatment with tamoxifen, a compound with both estrogen-like and anti-estrogen properties (a selective estrogen-receptor modulator).5 A meta-analysis of 55 studies evaluating tamoxifen for the treatment of women with breast cancer found that the drug was associated with an approximately 50 percent reduction in the risk for developing new cancers in the opposite breast among women who took the drug for five years.6

The USPSTF found and evaluated four randomized controlled trials (RCTs) of breast cancer chemoprevention in women who had never had breast cancer.5 Three of these trials used tamoxifen as the chemopreventive agent79; one trial used raloxifene, another selective estrogen-receptor modulator.10

Of the three RCTs of tamoxifen, the largest (the Breast Cancer Prevention Trial [BCPT]), with 13,388 women enrolled, found a risk reduction of invasive cancer of 49 percent among women at high risk for breast cancer (estimated five-year risk of 1.66 percent or greater).8 Over the course of the BCPT, a total of 264 women were diagnosed with invasive breast cancer: 175 in the placebo group and 89 in the tamoxifen group (relative risk [RR], 0.51; 95 percent confidence interval [CI], 0.39 to 0.66). The absolute risk reduction was 21.4 cases per 1,000 women over five years.

The two other tamoxifen RCTs did not show a similar benefit. The relative risk reduction for breast cancer was 0.94 (95 percent CI, 0.59 to 1.43) for the Royal Marsden Hospital study7 and 0.87 (95 percent CI, 0.62 to 2.14) for the Italian Tamoxifen Prevention Study.9 Although the reasons for these discrepant results are not definitively established, possible explanations include differences in the duration of therapy and differences between women enrolled in each study.1 The average duration of therapy was shorter in the European trials and, compared with the women enrolled in BCPT, the women in these trials were younger, had more estrogen-receptor-negative cancers, and were more likely to be taking hormone replacement therapy or to have had an oophorectomy.1

The study evaluating raloxifene in post-menopausal women with osteoporosis found a 76 percent risk reduction (RR, 0.24; 95 percent CI, 0.13 to 0.44) in the development of invasive breast cancer.10 After a median follow-up of 40 months, the absolute risk reduction among women taking raloxifene was 7.9 cases per 1,000 women (number needed to treat, 126).10 When effective, both raloxifene and tamoxifen were effective only against estrogen-receptor-positive tumors.1


Both tamoxifen and raloxifene increase the risk for thromboembolic events and hot flashes; tamoxifen increases the risk for endometrial cancer.1 The number of total thromboembolic events in all four trials was small, and differences in specific complication rates between the treatment and placebo arms were statistically significant only for pulmonary embolism.1 Among women aged 50 and older, for whom the potential harms of tamoxifen and raloxifene are more common than they are for younger women, the BCPT reported that after a median of 55 months of use, tamoxifen increased the rate of stroke from 1.3 cases per 1,000 women in the placebo group to 2.2 cases per 1,000 women in the study group (RR, 1.75; 95 percent CI, 0.98 to 3.20); increased the rate of pulmonary embolism from 0.3 cases per 1,000 women in the placebo group to 1.0 cases per 1,000 women in the study group (RR, 3.19; 95 percent CI, 1.12 to 11.15); and increased the rate of deep-vein thrombosis from 0.9 cases per 1,000 women in the placebo group to 1.5 cases per 1,000 women in the study group (RR, 1.71; 95 percent CI, 0.85 to 3.58).8

Fewer thromboembolic events occurred among women younger than 50, and the trial found no significant difference in incidence between the tamoxifen and placebo groups in this age group.8 The relative risk increase in venous thromboembolism from tamoxifen or raloxifene appears similar to the risk for venous thromboembolism from oral contraceptives or hormone replacement therapy.1

Among women aged 50 and older in the BCPT, participants who received tamoxifen, compared with those who took placebo, had a 4.0 times greater risk (95 percent CI, 1.70 to 10.90) of developing stage 1 endometrial cancer (0.8 cancers per 1,000 women taking placebo vs. 3.1 cancers per 1,000 women taking tamoxifen for a median of 55 months).8 Among women younger than 50, the BCPT found no significant difference in endometrial cancer rates between the two groups. No deaths attributed to endometrial cancer occurred in the trial.8 Raloxifene has not been associated with an increase in endometrial cancer.10

The BCPT reported that women in the tamoxifen group were at increased risk for developing cataracts and having cataract surgery compared with placebo (RR, 1.14 [95% CI, 1.01 to 1.29] and 1.57 [95% CI, 1.16 to 2.14], respectively).8

Quality-of-life issues have also been of concern and were addressed in the BCPT. Women in the BCPT reported increased rates of bothersome hot flashes (45.7 percent in the tamoxifen group vs. 28.7 percent in the placebo group) and bothersome vaginal discharge (12.4 percent in the tamoxifen group vs. 4.5 percent in the placebo group).8 Women given raloxifene also noted higher rates of hot flashes than women given placebo (10.7 percent in the raloxifene group vs. 6.4 percent in the placebo group).10

Although long-term adherence for highly motivated women was about 80 percent in the BCPT and about 90 percent in the raloxifene trial, adherence rates in the general population are unknown.2

The brief review of recommendations of others that is normally included in USPSTF recommendation statements is available in the complete Recommendation and Rationale statement on the USPSTF Web site (

This series is coordinated by Kenny Lin, MD, MPH, deputy editor.

A collection of USPSTF recommendation statements published in AFP is available at

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