Is there a link between Mirena (levonorgestrel) IUD and breast cancer?

Mirena IUD and Breast Cancer Risk

The Mirena (levonorgestrel-releasing) IUD is associated with a small but statistically significant increased risk of breast cancer (approximately 13% relative risk increase), and should not be used in women with current breast cancer; however, for women without breast cancer history, this modest risk must be weighed against substantial protective effects for endometrial and ovarian cancers.

Contraindications for Current Breast Cancer

Women with current breast cancer should absolutely not use the Mirena IUD 1, 2, 3. The CDC U.S. Medical Eligibility Criteria classifies current breast cancer as Category 4 (unacceptable health risk) for LNG-IUD use 1, 2. This is a firm contraindication because breast cancer is a hormonally sensitive tumor, and the levonorgestrel released systemically could theoretically promote disease progression 1.

For women with past breast cancer and no evidence of disease for 5 years, the LNG-IUD is Category 3 (theoretical risks usually outweigh advantages), meaning it should generally be avoided except in exceptional circumstances with careful counseling 1, 2.

Quantifying the Breast Cancer Risk in Healthy Women

The most recent and highest quality evidence comes from a 2024 Swedish national cohort study of over 514,000 women, which found that LNG-IUD use was associated with a 13% increased risk of breast cancer (adjusted HR 1.13,95% CI 1.10-1.17) 4. This translates to approximately 1.63 additional breast cancer cases per 10,000 person-years 4.

To contextualize this risk:

• Baseline breast cancer incidence in reproductive-age women (20-49 years) is approximately 72 per 100,000 women annually 1, 2
• The absolute risk increase is small, though statistically significant 4
• A 2020 meta-analysis corroborated these findings, showing increased breast cancer risk with OR 1.16 (95% CI 1.06-1.28) for all women, with higher risk in women ≥50 years (OR 1.52) 5

Critical Risk Modifier: Family History

Women with a family history of breast cancer face substantially higher risk when using the LNG-IUD 4. The 2024 Swedish study demonstrated a significant additive interaction between LNG-IUD use and family history of breast cancer, with a 19% relative excess risk for interaction and 1.63 additional cases per 10,000 person-years specifically in this subgroup 4. This means the risk is not simply additive but synergistic in women with familial predisposition.

Protective Effects Against Other Cancers

The risk-benefit calculation must account for significant protective effects:

• 33% reduced risk of endometrial cancer (adjusted HR 0.67,95% CI 0.56-0.80) 4
• 14% reduced risk of ovarian cancer (adjusted HR 0.86,95% CI 0.75-0.99) 4
• 9% reduced risk of cervical cancer (adjusted HR 0.91,95% CI 0.84-0.99) 4

An earlier Norwegian cohort study of 104,318 women found even more dramatic protective effects: 47% reduction in ovarian cancer risk (RR 0.53) and 78% reduction in endometrial cancer risk (RR 0.22), with no increased breast cancer risk detected in that population 6. However, the more recent and larger 2024 Swedish study should take precedence given its superior methodology and sample size 4.

Clinical Decision Algorithm

For women WITHOUT personal or family history of breast cancer:

• The LNG-IUD can be used (Category 1 for family history alone per older guidelines, though this predates 2024 data) 2
• Counsel about the small absolute risk increase (approximately 13% relative risk)
• Emphasize substantial protective effects against endometrial and ovarian cancers
• Consider alternative contraception if patient has multiple other breast cancer risk factors (obesity, nulliparity, late first pregnancy)

For women WITH family history of breast cancer:

• Exercise heightened caution given the synergistic risk interaction 4
• Strongly consider non-hormonal alternatives (copper IUD is Category 1 with no restrictions) 1, 3
• If LNG-IUD is chosen, ensure close surveillance for breast cancer development 4

For women WITH current breast cancer:

• Absolute contraindication - do not use 1, 2, 3
• Recommend copper IUD as first-line alternative (Category 1) 3

For women WITH past breast cancer (>5 years disease-free):

• Generally avoid (Category 3) 1, 2
• Consider only after multidisciplinary discussion and informed consent 7
• Copper IUD remains the preferred option 3

Special Consideration: Tamoxifen Users

For women on tamoxifen therapy, the situation is nuanced. The LNG-IUD is recommended as first-line contraception by some guidelines specifically to protect against tamoxifen-induced endometrial changes 3. However, concerns exist about potential breast cancer recurrence, though a meta-analysis found no increased recurrence risk (RR 0.12,95% CI 0.02-0.91) 3. The evidence remains controversial and insufficient 3, 8.

For tamoxifen users, prioritize non-hormonal copper IUD unless significant endometrial pathology develops, in which case LNG-IUD may be considered after thorough counseling 3.

Common Pitfalls to Avoid

• Do not dismiss the breast cancer risk as clinically insignificant without discussing it with patients, particularly those with family history 4
• Do not assume screening asymptomatic women with breast examination before IUD insertion is necessary (low yield given low baseline incidence) 1
• Do not use LNG-IUD in women with current breast cancer under any circumstances 1, 2, 3
• Do not fail to counsel about the protective effects against other gynecologic cancers when discussing breast cancer risk 4