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Breast Cancer Prevention (PDQ®): Prevention - Health Professional Information [NCI] - Overview

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Magnitude of Effect: The RR for women consuming approximately four alcoholic drinks per day compared with nondrinkers is 1.32 (95% CI, 1.19–1.45). The RR increases by 7% (95% CI, 5.5%–8.7%) for each drink per day.

Study Design: Case-control and cohort studies.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Major inheritance susceptibility

Based on solid evidence, women who inherit gene mutations associated with breast cancer have an increased risk.

Magnitude of Effect: Variable, depending on gene mutation, family history, and other risk factors affecting gene expression.

Study Design: Cohort or case-control studies.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Factors Associated With Decreased Risk of Breast Cancer

Estrogen-only use among women with a hysterectomy

Based on fair evidence, estrogen-only use after menopause among women with a hysterectomy is associated with a decreased risk of breast cancer incidence and mortality.

Magnitude of effect: Based on one RCT of estrogen-only therapy with conjugated equine estrogen, there was a 23% decrease in incidence of invasive breast cancer (0.27% per year with a median of 5.9 years of use, compared with 0.35% per year among those taking a placebo).

Study Design: RCT and cohort studies.
Internal Validity: Good.
Consistency: Mixed.
External Validity: Good.

Exercise

Based on solid evidence, exercising strenuously for more than 4 hours per week is associated with reduced breast cancer risk.

Magnitude of Effect: Average RR reduction is 30% to 40%. The effect may be greatest for premenopausal women of normal or low body weight.

Study Design: Prospective observational and case-control studies.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Early pregnancy

Based on solid evidence, women who have a full-term pregnancy before age 20 years have decreased breast cancer risk.

Magnitude of Effect: 50% decrease in breast cancer compared with nulliparous women or those who give birth after age 35 years.

Study Design: Cohort and case-control studies.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Breast-feeding

Based on solid evidence, women who breast-feed have a decreased risk of breast cancer.

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