To investigate the relation of various estrogen-containing hormone therapies to the risk of breast cancer, emphasizing the use of the combination of estrogen and testosterone.
Using information from a large U.S.–based claims database, we conducted a case–control study in women aged 50 to 64 years who had a first-time diagnosis of breast cancer to estimate the effect in users of conjugated estrogen alone, conjugated estrogen plus progestin, esterified estrogen with methyltestosterone, or esterified estrogen with methyltestosterone plus progestin, compared with nonusers. Four controls were matched to each case on year of birth and index date. Odds ratios (ORs) were calculated using conditional logistic regression.
We identified 4,515 cases and 18,058 matched controls. The OR for users of estrogen alone compared with the nonusers was 0.96 (95% confidence interval [CI] 0.88–1.06; 667 cases and 2,900 controls); for users of conjugated estrogen plus progestin, it was 1.44 (95% CI 1.31–1.58; 712 cases and 2,087 controls); and for users of esterified estrogen with methyltestosterone and esterified estrogen with methyltestosterone plus progestin, the ORs were 1.08 (95% CI 0.86–1.36; 98 cases and 380 controls) and 1.69 (95% CI 1.03–2.79; 22 cases and 55 controls), respectively. There was an increased risk among conjugated estrogen plus progestin users of 48 months or more (OR 3.10, 95% CI 2.38– 4.04; 111 cases and 149 controls).
There is no materially increased risk of breast cancer in users of estrogen alone or esterified estrogen with methyltestosterone compared with nonusers. There is an increased risk among those using conjugated estrogen plus progestin. In particular, the risk of breast cancer in women who used conjugated estrogen plus progestin for 4 or more years is approximately three times higher than in women who are not exposed to hormone therapy, so that the background incidence rate for women aged 50 to 64 years, which is around 3 per 1,000, would be increased to approximately 9 per 1,000 in women aged 50 to 64 years who have taken conjugated estrogen plus progestin for 48 months or more.
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