The Texas Supreme Court recently held that plaintiffs seeking to prove general causation with epidemiological evidence must produce two independent studies demonstrating that subjects who used the product at issue under circumstances substantially similar to those encountered by the plaintiff doubled their the risk of injury. Merck & Co. v. Garza, 2011 WL 3796364 (Tex. Aug. 26, 2011).

Leonel Garza, a 71-year old with a 20-year history of heart disease, died of a heart attack less than one month after he began taking the anti-inflammatory drug Vioxx. Garza’s family sued Merck, the manufacturer of Vioxx, alleging defective design and inadequate warnings. A jury awarded the Garzas $7 million in actual damages and $25 million in punitive damages (reduced to $750,000).

The court of appeals held that plaintiffs introduced sufficient evidence that Vioxx caused Garza’s death, rejecting Merck’s argument that plaintiffs’ causation evidence was inadequate under the standard adopted in Merrell Dow Pharmaceuticals, Inc. v. Havner, 953 S.W.2d 706 (Tex. 1997). In Havner, the Texas Supreme Court held that a causation opinion based on epidemiological evidence must be supported by statistically-significant studies showing a “doubling of the risk” of injury at a comparable dose and duration to that experienced by the plaintiff. The Havner court observed, however, that “a relative risk of more than 2.0” is not a “litmus test” and that trial judges should consider the “totality of the evidence.” Citing this “totality of the evidence” standard, the court of appeals in Garza held that Havner did not establish a “bright-line test.”

The Texas Supreme Court reversed, holding that when plaintiffs “attempt to prove general causation using epidemiological evidence,” plaintiffs must produce two independent studies finding “a statistically significant doubling of the risk” of injury for patients taking the drug, show that plaintiffs are “similar to the subjects in the studies” in terms of dosage and duration, and demonstrate that “other plausible causes of the injury or condition that could be negated are excluded with reasonable certainty.” The court stated that only if plaintiffs satisfy these “threshold requirements” must the trial judge “examine[] the soundness” of the studies under a “totality of the evidence” standard.

In so holding, the Texas Supreme Court rejected plaintiffs’ attempt to distinguish Havner as a case involving observational studies, which are less reliable than clinical trials. According to the court, because “[t]he purpose of the structure of epidemiological studies and the statistical evaluation of their results is to provide objective criteria by which it can be determined with reasonable certainty that a particular individual’s injury was caused by exposure to a given substance,” “Havner’s requirements necessarily apply to all epidemiological evidence.”

The court also rejected plaintiffs’ argument that they presented two studies showing a statistically significant doubling of the risk of heart attack from taking Vioxx. The court found that the conditions in plaintiffs’ studies were not “substantially similar” to Garza’s circumstances because he “took a much smaller dosage of Vioxx for much less time.”

Garza is of obvious importance to cases filed in Texas courts, as it clarifies that the “totality of the evidence” standard identified in Havner does not allow plaintiffs seeking to prove causation with epidemiological evidence to evade Havner’s requirement that they produce two independent studies finding a statistically significant doubling of the risk of injury. Garza’s tightening of causation standards also may prove influential in courts outside Texas that have not yet identified a standard governing the use of epidemiological evidence to prove causation.