Reassessing the Mirena Breast-Cancer Lawsuit: Legal Developments & Challenges Since 2017

When evaluating the legal landscape in the U.S. regarding the contraceptive intrauterine device (IUD) Mirena, manufactured by Bayer AG, one of the most significant and emerging strands involves claims of an elevated risk of breast cancer. This article, from a U.S. product‐liability law perspective, examines the key issues, developments since 2017, and the legal challenges surrounding what is often referred to as the “Mirena breast cancer lawsuit.”

Mirena & The Alleged Breast Cancer Risk

Mirena is a T-shaped intrauterine device that releases the hormone levonorgestrel, approved in the United States for both contraception and heavy menstrual bleeding. Some legal claims assert that the hormone exposure via Mirena may contribute to increased breast‐cancer risk—but the scientific, regulatory, and legal paths have been complex.

In 2014, Finnish researchers published a study in Obstetrics & Gynecology indicating that women who used levonorgestrel‐releasing IUDs (such as Mirena) to treat heavy menstrual bleeding might face a ~20% higher risk of breast cancer. Also, recent large studies (e.g., in Denmark) show a ~40% increased breast‐cancer risk among certain users of levonorgestrel IUDs (not Mirena‐specific) compared to non-hormonal IUD users.

These data underpin many of the legal claims alleging that Bayer should have warned patients and physicians or altered product labelling to reflect the risk.

Legal Claims & The 2017 Context

Although many of the major MDLs (multidistrict litigations) involving Mirena (for uterine perforation, device migration, pseudotumor cerebri) were concentrated before and around 2017, the breast cancer dimension was emerging.

• By 2017, Bayer had offered a ~US$12.2 million settlement for thousands of claims regarding migration/perforation injuries—but not for breast cancer claims.
• The breast cancer claims were not yet consolidated in the same way or widely resolved by 2017. Many of these claims were still at early stages.
• A key development: In 2017, courts scrutinised whether allowable claims could proceed. While many of the device‐migration/pseudotumor cerebri cases were being dismissed for lack of causation or statute‐of‐limitations, the breast‐cancer claims faced similar legal headwinds. For example, a 2017 order discussed Bayer’s “2017 signal assessment” of adverse events.

Thus, while 2017 is often cited as a marker year in the Mirena litigation timeline, the breast‐cancer subset of litigation was still nascent and largely unresolved by that date.

The Legal Theories in Play

In a typical “Mirena breast-cancer” claim, the legal theories include:

1. Failure to warn / inadequate labelling – Plaintiffs allege Bayer knew or should have known about the elevated breast-cancer risk and failed to adequately disclose it to prescribers and patients. For example, the class‐action complaint filed in California (March 2022) asserts that Bayer omitted disclosure of a “significantly increased risk of breast cancer” in Mirena users.
2. Negligence, strict liability – Plaintiffs may allege that the device was defectively designed or that the warnings were inadequate, giving rise to product-liability liability under state law.
3. Consumer-fraud / misrepresentation – Some claims frame the issue as misleading marketing (i.e., that Bayer marketed Mirena as safe without adequate disclosure of cancer risk).

However, as with many complex medical‐device litigation matters, a key hurdle is proving causation (that Mirena caused or materially increased the risk of breast cancer in a particular user) and establishing the manufacturer’s knowledge or ability to have warned.

Developments After 2017 and Current Status

Since 2017 the litigation has evolved:

• In October 2023, a U.S. federal judge cleared a class‐action lawsuit alleging Mirena caused breast cancer to proceed past the motion to dismiss stage, finding that Bayer’s argument regarding scientific causation and federal preemption raised questions for later proceedings.
• However, as of early 2024, some such claims have been voluntarily dismissed by the parties (e.g., a stipulation of dismissal in May 2024) even though the underlying scientific research continues to evolve.
• A major Danish cohort study (e.g., women aged 15-49 using levonorgestrel IUDs) found about a 1.4–1.8× increased breast cancer risk over 6.8 years versus non‐hormonal IUD users—not Mirena‐specific, but relevant to the legal argument.
• Legal commentary notes that many lawyers are not accepting new Mirena breast cancer or migration/perforation cases, especially given the closure of MDLs and expiration of statutes of limitation in many states.

Key Legal Challenges

For plaintiffs in a Mirena breast‐cancer lawsuit, the following difficulties are especially significant:

• General causation: Showing that Mirena (or the hormone levonorgestrel delivered via Mirena) is capable of causing breast cancer at the level alleged. Courts apply rigorous review of experts under Daubert/FRE 702 standards.
• Specific causation: For a particular plaintiff, showing that the device caused or materially contributed to her breast cancer (vs. other risk factors).
• Statute of limitations / repose: Many claims may be barred if filed too late under state law. Unlike device migration claims filed earlier, breast cancer litigation may bring different time‐limits depending on jurisdiction.
• Preemption / regulatory defence: Bayer may assert that federal FDA regulation and labelling preempt certain state law claims (though in the 2023 order, the court rejected most preemption arguments at the motion stage)
• Risk magnitude: Though some studies indicate elevated risk, the absolute risk remains relatively low—raising potential challenges on damages and whether warning would have changed the use decision. For example, a study noted 14 additional breast cancers per 10,000 women over five years among IUD users.

Implications and Advice for Potential Claimants

If someone (or a legal advisor) is evaluating a possible claim alleging Mirena caused breast cancer, the following practical points apply:

• Time is of the essence—check your jurisdiction’s applicable statute of limitations or repose period.
• Preserve all medical records including insertion/removal records, diagnosis date of breast cancer (or precancerous condition), implant timing, and any exposure data.
• Document other known breast‐cancer risk factors (family history, hormonal therapies, lifestyle) because these will likely be contested.
• Just because a study shows increased risk for levonorgestrel IUDs doesn’t automatically mean Mirena caused a plaintiff’s individual cancer—each case must bridge causation.
• Consider whether multiple jurisdictions may apply (state law claims, federal class action, etc.).
• Legal counsel with experience in medical device product liability is essential, given the scientific, regulatory and procedural complexity.

Conclusion

Although by 2017 many of the large‐scale device migration/perforation litigations involving Mirena had advanced significantly, the breast cancer dimension remained emerging. Over time, more scientific studies and legal filings have brought the issue into sharper focus. Still, plaintiffs face steep legal hurdles—especially causation, limitations timing, and risk magnitude. For women who used Mirena and later developed breast cancer, these lawsuits offer one potential avenue for redress—but they also highlight the high burden of proof in modern medical-device litigation.