Does hormone replacement therapy (HRT) increase the risk of cancer?

Does Hormone Replacement Therapy Increase Cancer Risk?

Yes, hormone replacement therapy (HRT) increases the risk of certain cancers, particularly breast cancer with combined estrogen-progestin therapy and endometrial cancer with unopposed estrogen, though it reduces colorectal cancer risk. The cancer risk profile differs substantially based on the type of HRT used, duration of therapy, and individual patient characteristics.

Cancer Risks by HRT Type

Combined Estrogen-Progestin Therapy

Combined estrogen-progestin therapy carries good evidence for increased breast cancer risk 1. Based on the landmark Women's Health Initiative (WHI) data, for every 10,000 women taking combined therapy for one year, there are 8 additional invasive breast cancers 1. This risk:

• Increases proportionally with duration of use 2
• Becomes statistically significant after extended follow-up (11 years showed clear increased risk) 3
• Decreases relatively quickly after discontinuation, with the increased risk dissipating within 2 years 4
• Appears higher when initiated close to menopause rather than later 5

The type of progestin matters significantly. Synthetic progestins carry higher breast cancer risk (RR 1.4) compared to micronized progesterone (RR 0.9), even with short-term use 6. This is a critical clinical distinction that should guide prescribing decisions.

Combined therapy does not increase endometrial cancer risk—in fact, cohort studies show decreased risk (RR 0.4) 7, 1, 7. The WHI confirmed no increased endometrial cancer with combined therapy 7.

Unopposed Estrogen Therapy

Unopposed estrogen dramatically increases endometrial cancer risk with good evidence 1. The relative risk is 2.3 compared to nonusers, escalating to RR 9.5 with 10 years of use 7, 1. This risk remains elevated 5+ years after discontinuation 1.

For breast cancer, the evidence with unopposed estrogen is more nuanced. The WHI trial of estrogen-alone actually showed decreased breast cancer risk after 11 years of follow-up 3, contrasting with earlier concerns. However, this applies specifically to women who have had hysterectomies.

Other Cancer Considerations

HRT reduces colorectal cancer risk with fair evidence—6 fewer cases per 10,000 women per year with combined therapy 1. This benefit must be weighed against the increased risks.

Ovarian cancer evidence is insufficient and inconsistent 1, 7, 1. Some cohort studies suggest increased risk (RR 1.8-2.2) with 10+ years of use, but data cannot determine definitive effects 7.

Risk Stratification Factors

The HRT-related cancer risk is not uniform across all women:

• Body weight: Leaner women have higher HRT-related breast cancer risk 2
• Breast density: Women with dense breasts face higher risk 2
• Age at initiation: Starting HRT after age 45 may carry more than 3-fold increased breast cancer risk compared to starting earlier 8
• Obesity status: The protective effect of combined HRT against endometrial cancer is greatest in obese women 9

Clinical Decision-Making Algorithm

For women considering HRT:

1. Determine if patient has intact uterus:

   • If yes → Combined estrogen-progestin required (never unopposed estrogen due to endometrial cancer risk)
   • If no (post-hysterectomy) → Estrogen alone is appropriate and may actually reduce breast cancer risk

2. If combined therapy needed, select progestin type:

   • Prefer micronized progesterone over synthetic progestins to minimize breast cancer risk 6
   • Consider transdermal estrogen in those with VTE risk factors 8

3. Assess individual cancer risk factors:

   • High breast density, lean body mass, age >45 at initiation → Higher breast cancer risk
   • Previous breast cancer → HRT generally contraindicated 8
   • BRCA carriers <45 years → HRT can be offered for up to 4 years without clear increased breast cancer risk 8

4. Duration considerations:

   • Breast cancer risk increases with duration, particularly beyond 5 years 1, 10
   • Reassess continuation at least annually 10
   • Use lowest effective dose for shortest duration consistent with treatment goals 1, 7

Absolute Risk Context

The absolute increases in cancer risk are modest. Per 10,000 women taking combined estrogen-progestin for one year: 8 more breast cancers, but also 6 fewer colorectal cancers 1. However, when adding breast and endometrial cancers together, combined HRT causes greater total cancer incidence than other therapies 9.

Critical Caveats

• Most evidence comes from conjugated equine estrogen (CEE) and medroxyprogesterone acetate (MPA); other formulations may have different risk profiles 1, 7
• The WHI studied older women (average age 63); risks may differ in younger women with premature menopause 8
• For women with premature ovarian insufficiency or early menopause (<45 years), HRT should be offered until age 50 as benefits likely outweigh risks in this population 8, 10
• Evidence beyond 5-7 years of use is limited 10

The decision requires shared decision-making, weighing individual cancer risks, menopausal symptom severity, fracture risk, cardiovascular risk factors, and patient preferences 1.