From Possibility to Probability: Sixth Circuit Clarifies Causation Standard in Tennessee Products-Liability Actions

Introduction

This commentary examines the Sixth Circuit’s unpublished opinion in Cyndi Buchanan v. Johnson & Johnson Consumer, Inc. (6th Cir. Aug. 4, 2025). The judgment reinforces a key evidentiary principle in Tennessee products-liability litigation: a plaintiff must adduce probative, non-speculative evidence that a product defect was a “substantial factor” in producing the claimed injury. Merely showing that the product “cannot be ruled out” is insufficient to defeat summary judgment.

The dispute arose after Cyndi Buchanan experienced total body hair loss (alopecia universalis) and attributed it to DMDM hydantoin—a preservative alleged to be present in Johnson & Johnson’s OGX Biotin & Collagen shampoo and conditioner. The district court granted summary judgment for the manufacturer, and the Court of Appeals affirmed.

Summary of the Judgment

Key Holding: A plaintiff cannot survive summary judgment on a Tennessee Products Liability Act (TPLA) claim when her expert opines only that the product could have caused the injury or “cannot be ruled out” as a cause. Evidence must permit a reasonable juror to find that the defect was a “substantial factor” in causing the harm.
Disposition: Affirmed – summary judgment for Johnson & Johnson.
Central Rationale: Buchanan’s expert testimony was speculative; non-expert evidence was insufficient; consequently, no genuine dispute of material fact existed regarding causation.
Erie Considerations: The panel found it unnecessary to decide whether Tennessee’s “expert testimony requirement” is substantive or procedural because the claim failed even when all evidence (expert and lay) was considered.

Analysis

1. Precedents Cited and Their Influence

a) Sigler v. American Honda Motor Co., 532 F.3d 469 (6th Cir. 2008)
– Sets out the three-prong test for TPLA claims: defect, existence of defect at time of manufacture, and proximate cause.

b) Jackson v. Ford Motor Co., 842 F.3d 902 (6th Cir. 2016)
– Clarifies that under the TPLA, a defect must be a “substantial factor” in causing injury.

c) Primm v. Wickes Lumber Co., 845 S.W.2d 768 (Tenn. Ct. App. 1992) & Lindsey v. Miami Dev. Corp., 689 S.W.2d 856 (Tenn. 1985)
– Tennessee cases rejecting expert opinions that amount to speculation.

d) Boyd v. Baeppler, 215 F.3d 594 (6th Cir. 2000)
– Sixth Circuit precedent that speculative expert testimony cannot create a genuine issue of fact.

e) Erie R.R. Co. v. Tompkins, 304 U.S. 64 (1938) & Madej v. Maiden, 951 F.3d 364 (6th Cir. 2020)
– Frame the substantive/procedural inquiry when applying state evidentiary rules in federal diversity cases.

These authorities collectively informed the panel’s view that causation—rather than defect identification—was dispositive, and that speculation cannot meet the “substantial factor” threshold demanded by Tennessee law and Rule 56 alike.

2. Legal Reasoning of the Court

Application of TPLA Elements. The panel assumed arguendo that Buchanan’s product contained DMDM, focusing exclusively on the third Sigler element—proximate cause.
Assessment of Expert Testimony. Dr. Schwartz repeatedly stated he could not rule DMDM “in” as a cause. The court characterised such testimony as possibility, not probability, invoking Boyd to deem it insufficient.
Consideration of Lay/Documentary Evidence. Even if Tennessee’s “expert-only in complex medical cases” requirement were procedural (and thus inapplicable), the non-expert materials (FDA notices, advocacy letters, complaints statistics) did not establish that DMDM played a substantial causal role in Buchanan’s unique medical progression.
Rule 56 Standard. Absent evidence from which a reasonable juror could infer more than a “mere possibility,” summary judgment was mandatory.
Erie Bypass. Because Buchanan’s evidence failed under both state and federal standards, the panel declined to resolve whether Tennessee’s expert-testimony rule is substantive.

3. Potential Impact of the Judgment

Heightened Evidentiary Scrutiny. Plaintiffs in the Sixth Circuit bringing TPLA (or analogous state-law) claims must now anticipate closer judicial scrutiny of expert phrasing. “Cannot rule out” will be treated as speculative.
Guidance on Complex Medical Causation. The case signals that even where an Erie conflict is debatable, federal courts may sidestep it by asking whether all evidence—expert and lay—could allow a reasonable jury to find causation.
Manufacturers’ Litigation Strategy. Defendants can succeed on summary judgment by attacking the probative value of expert opinions rather than disputing every factual element (e.g., defect existence).
Expert Witness Preparation. Medical experts must commit to an opinion framed in probabilistic, causative terms (e.g., “more likely than not”) rather than mere non-exclusion.
Influence Beyond Tennessee. While grounded in Tennessee law, the decision may be cited in other jurisdictions for the proposition that speculative causation evidence is insufficient, reinforcing a trend across federal circuits.

Complex Concepts Simplified

TPLA (Tennessee Products Liability Act):
A statute providing the exclusive framework for recovering damages caused by defective or unreasonably dangerous products in Tennessee.

Substantial Factor:
A cause that is more than a remote or trivial contributor to the injury; it must play a significant role in bringing about the harm.

Alopecia Areata vs. Alopecia Universalis:
– Alopecia areata: Autoimmune condition causing patchy hair loss.
– Alopecia universalis: Advanced form leading to complete hair loss over the entire body.

DMDM Hydantoin:
Cosmetic preservative that releases small amounts of formaldehyde when water is present; alleged to irritate skin/hair follicles.

Erie Doctrine:
Federal courts hearing state-law claims must apply state substantive law and federal procedural law. Determining whether a rule is substantive or procedural affects which law controls.

Rule 56 (Federal Rules of Civil Procedure):
Governs summary judgment; permits judgment without trial when no genuine dispute of material fact exists.

Conclusion

The Sixth Circuit’s opinion in Buchanan v. Johnson & Johnson Consumer, Inc. underscores the line between possibility and probability in products-liability causation proofs. By holding that an expert’s inability to “rule out” a cause is inadequate, the court tightened the evidentiary belt for plaintiffs alleging complex medical injuries. Going forward, litigants must marshal expert testimony that the alleged defect was more likely than not a substantial factor in the injury or risk dismissal at the summary-judgment stage. This decision thus harmonises Tennessee precedent with federal summary-judgment standards and serves as a cautionary tale for future plaintiffs and experts alike.