Recent discussions have raised questions about a potential link between levonorgestrel-releasing IUDs (like the Mirena®) and an increased breast cancer risk. If you're concerned by these reports, it's helpful to take a closer look at the data behind the claims to make an informed decision.
Study Findings Explained
A Danish study tracked approximately 8,000 women aged 15 - 49 Years and found that users of levonorgestrel IUDs showed a 40% higher relative risk of developing breast cancer. However, this sounds much more alarming than it is. When converted into absolute risk, this increase corresponds to 14 additional cases of breast cancer per 10,000 women—an increase of less than 1%. While the relative percentage may seem significant, the actual increase in individual risk remains small.
Study Limitations: Why Caution is Needed
It’s important to note that this study was observational, meaning it relied on pre-collected data from health registries. While observational studies can reveal trends, they are not as reliable as randomized controlled trials (RCTs), which can more effectively account for various influencing factors.
This study did not fully adjust for critical lifestyle factors such as smoking, alcohol use, physical activity, or obesity, all of which are known to increase breast cancer risk. This raises the possibility that other influences may have skewed the results. Interestingly, previous clinical trials have not found significant breast cancer risks in women using levonorgestrel IUDs, suggesting that more rigorous studies are needed to confirm these recent findings.
How Levonorgestrel IUDs Function
Levonorgestrel IUDs primarily release small doses of progestogen directly into the uterus, with only minimal levels entering the bloodstream. This makes them highly effective as both contraceptives and tools for regulating menstrual cycles. For women using menopausal hormone therapy (MHT), Mirena® can also protect the endometrium from overgrowth, offering additional health benefits.
The research suggests that the slight increase in breast cancer risk may be related to progestogen’s effect on accelerating the growth of preexisting cancer cells, rather than creating new cancer cells. At the same time, levonorgestrel IUDs are associated with reduced risks of other cancers, such as endometrial, ovarian, and cervical cancers, as well as fewer menstrual-related problems.
Balancing Risks with Benefits
For most women, the benefits of using a levonorgestrel IUD far outweigh any small increase in breast cancer risk. The risk remains very low in absolute terms, and the IUD offers a range of valuable health advantages, including long-term contraception, menstrual control, and endometrial protection.
That said, every woman’s situation is unique, and it’s important to consult with a healthcare provider to determine the best contraceptive option based on personal health needs and medical history.
Final Thoughts
While some studies suggest a possible slight increase in breast cancer risk with levonorgestrel IUDs, the overall risk remains minimal. At the same time, the IUD offers a wide range of benefits that make it a reliable and safe choice for many women. Moving forward, additional high quality research will be necessary to better understand the relationship between hormonal IUDs and breast cancer.
If you have any concerns, talk to your doctor for personalised medical advice to help guide your decision making.
Dr Karen Osborne, BSc (Hons), MBBS, MRCGP, DRCOG, DFSRH, FRACGP
Clinical Director, Clinic 66
References:
- Mørch, L. S., Meaidi, A., Corn, G., et al. (2024). Breast Cancer in Users of Levonorgestrel-Releasing Intrauterine Systems. JAMA, Published online October 16, 2024. doi:10.1001/jama.2024.18575.
- Yi, H., Zhang, N., Huang, J., et al. (2024). Association of Levonorgestrel-Releasing Intrauterine Device with Gynecologic and Breast Cancers: A National Cohort Study in Sweden. American Journal of Obstetrics & Gynecology, 231(450): e1-12.
- Levonorgestrel-Releasing Intrauterine System and Breast Cancer Risk: A Systematic Review and Meta-Analysis. Acta Obstetricia et Gynecologica Scandinavica, 99(8), 970-982. doi:10.1111/aogs.13817
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