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Despite reducing risk for invasive breast cancer, certain drugs increased risk for adverse events

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Tamoxifen, raloxifene and tibolone reduced the risk for invasive breast cancer by seven to 10 women per 1,000 women per year, according to meta-analysis findings. However, these drugs also increased the risk for adverse events, including thromboembolic events, endometrial cancer and stroke.

Researchers reviewed seven placebo-controlled randomized trials and one head-to-head trial selected from the Medline and Cochrane databases, Web of Science, trial registries and manufacturer information.

Data revealed that tamoxifen, raloxifene and tibolone reduced the incidence of invasive breast cancer from 30% to 68% in midlife and older women. The data were from four placebo-controlled trials of tamoxifen (RR=0.70; 95% CI, 0.59-0.82), two trials of raloxifene (RR=0.44; 95% CI, 0.27-0.71) and one trial of tibolone (RR=0.32; 95% CI, 0.13-0.80).

Tamoxifen and raloxifene reduced the risk for ER-positive but not ER-negative breast cancer or noninvasive cancer. All-cause mortality was similar for the three drugs compared with placebo, and tamoxifen did not demonstrate reduced breast cancer mortality compared with placebo (RR=1.07; 95% CI, 0.66-1.74). Data were not available for breast cancer mortality with raloxifene and tibolone. All medications reduced fracture risk.

Despite these reduced risks, the drugs increased the risk of other adverse events. Thromboembolic events increased by four to seven in 1,000 women per year with tamoxifen (RR=1.93; 95% CI, 1.41-2.64) and raloxifene (RR=1.60; 95% CI, 1.15-2.23). Compared with placebo, tamoxifen increased endometrial cancer cases by four in 1,000 women per year (RR=2.13; 95% CI, 1.36-3.32). Women using tamoxifen also reported more benign gynecologic conditions, surgical procedures including hysterectomy, and uterine bleeding compared with placebo. In the LIFT trial, tibolone caused more strokes than placebo; stroke occurred more often in women aged older than 70.

The researchers noted several limitations to this study, including trial heterogeneity, few head-to-head trials, and a lack of data on doses, duration, timing, long-term effects, and non-white or premenopausal women.

Nelson HD. Ann Intern Med. 2009;151.