Second Circuit Affirms Mirena MDL Court’s “Hard Look” at Plaintiffs’ Experts’ Methodology


On December 8, 2020, the Second Circuit Court of Appeals affirmed the Southern District of New York’s granting of summary judgment in favor of Bayer — and resulting closure of all cases against Bayer — in the Mirena multidistrict litigation (MDL). In re Mirena IUS Levonorgestrel-Related Prod. Liab. Litig. (No. II), No. 19-2155, 2020 WL 7214264 (2d Cir. Dec. 8, 2020).

In the MDL, the plaintiffs alleged that the Mirena Intrauterine System had caused them to develop idiopathic intracranial hypertension (IIH). The District Court elected to focus first on whether the plaintiffs had evidence sufficient to establish general causation. The District Court held a Daubert hearing that lasted three days and featured testimony by 19 general causation witnesses — 7 for the plaintiffs and 12 for Bayer. On October 24, 2018, the District Court entered a detailed 156-page opinion granting Bayer’s Daubert motion as to all of the plaintiffs’ experts and denying as moot plaintiffs’ motion to preclude Bayer’s experts. In re Mirena IUS Levonorgestrel-Related Prods. Liab. Litig., 341 F. Supp. 3d 213 (S.D.N.Y. 2018). Bayer then filed a motion for summary judgment, which the District Court granted for lack of general causation and dismissed all cases in the Mirena MDL.

On appeal, the plaintiffs argued that the District Court first erred by excluding the opinions of their experts on general causation and then erred again in granting summary judgment. The Second Circuit reviewed and affirmed both of the District Court’s decisions.


The Second Circuit began its analysis by noting that district courts are gatekeepers when it comes to the admission of expert testimony: they have to ensure that such testimony “both rests on a reliable foundation and is relevant to the task at hand.” Daubert v. Merrell Dow Pharm., 509 U.S. 579, 597 (1993). The plaintiffs argued that the District Court erred in this analysis in three different ways.

  • Plaintiffs first argued that the District Court erred by taking a “hard look” at their experts’ methodologies, which they claimed was “too searching.” The Second Circuit held that the District Court’s analysis was not only appropriate but also required to ensure reliability:

    “[A]n expert’s methodology must be reliable at every step of the way, and ‘[i]n deciding whether a step in an expert’s analysis is unreliable, the district court should undertake a rigorous examination of the facts on which the expert relies, the method by which the expert draws an opinion from those facts, and how the expert applies the facts and methods to the case at hand.’”

  • Plaintiffs next argued that the District Court impermissibly focused on their experts’ conclusions instead of their methodologies. The Second Circuit rejected this argument, quoting several portions of the District Court’s 156-page opinion that clearly focused on methodological flaws.
  • Finally, the plaintiffs argued that the District Court erred in requiring their experts to support their opinions with published studies. It bears noting that the plaintiffs’ experts had in fact cited two epidemiological studies as support for their opinions; however, one study had been repudiated by its own author for failing to control for important variables, and the other study expressly admitted it had found only a correlation, not causation. 341 F.Supp.3d at 229-35.The Second Circuit explained that, although an expert need not back his or her opinion with published studies that support his or her conclusion if he or she has used reliable scientific methods to reach that conclusion, the District Court had found that the plaintiffs’ experts did not use reliable scientific methods. Because their conclusions also were not supported by other studies, the experts’ testimony was properly excluded. The Second Circuit emphasized and concluded that the District Court “appropriately undertook a rigorous review” of the experts’ opinions, methodologies, and conclusions, and reasonably found that their methodologies were not sufficiently reliable, and their conclusions were not otherwise supported by the scientific community.

Motion for Summary Judgment

As the Second Circuit noted, state law controls what evidence is necessary to prove an element of a state law claim, such as general causation. The Mirena plaintiffs failed to point to any state that does not require a showing of general causation. However, they argued that the District Court had erred in three ways by excluding evidence with which they alleged that they could have established general causation.

First, the plaintiffs argued that the District Court should have excluded only portions of their experts’ reports instead of excluding them in their entirety. However, the plaintiffs failed to identify any portions of the reports that were admissible (notwithstanding that the Second Circuit had affirmed the District Court’s Daubert analysis) and sufficient to establish general causation. Accordingly, the Second Circuit declined to speculate that some unidentified evidence might have created a genuine dispute regarding general causation.

Next, the plaintiffs argued that the District Court erred in precluding differential-diagnosis evidence. Although the Second Circuit declined to hold that a differential diagnosis may never provide a sufficient basis to opine on general causation, it deferred to the District Court’s “broad discretion in excluding differential-diagnosis evidence.”

Finally, the plaintiffs argued that the District Court improperly precluded them from obtaining other general-causation evidence in discovery. The Second Circuit pointed to millions of documents from more than 50 custodians produced by Bayer. While most of these custodians had been identified in the previous Mirena MDL, the District Court allowed the plaintiffs to obtain documents from 11 more custodians. The Second Circuit concluded that the District Court’s thorough and well-reasoned discovery orders throughout the litigation were within its discretion.


Commentators have recently voiced concerns over the judiciary’s alleged relaxation of the gatekeeper role assigned to it by Daubert and its progeny. In the Mirena decision, the Second Circuit has reaffirmed that it is not merely appropriate but necessary for judges to take a “hard look” at an expert’s proffered testimony and ensure that the expert’s methodology is “reliable at every step of the way.”

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About the Author: Eric M. Friedman

Eric Friedman guides clients through all stages of product liability litigation, particularly working with expert witnesses to present the science behind clients' products. By leaning on his pre-law history as a biochemist, he is able to identify key arguments for and against clients and craft winning strategies for both motion practice and trial.

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