State v. Seymour: But-For Causation in Mixed-Drug Overdose Cases Does Not Require the Defendant’s Drug to Be the Sole Cause

1. Introduction

State v. Seymour (Supreme Court of Ohio, 2026-Ohio-1249) addresses causation in prosecutions arising from a mixed-drug overdose death. The State alleged that Carol A. Seymour furnished heroin to “Adam,” who died shortly after returning home. Toxicology and autopsy evidence showed multiple drugs in Adam’s system—heroin, mitragynine (kratom), methylphenidate, and diphenhydramine— which experts described as having a “synergistic effect.”

Seymour was convicted at a bench trial of involuntary manslaughter under R.C. 2903.04(A) and corrupting another with drugs under R.C. 2925.02(A)(3) (and also heroin trafficking, not at issue on appeal). The Tenth District reversed the two causation-dependent convictions on sufficiency grounds, reasoning there was no testimony that heroin was the “but-for” cause of death in a mixed-drug overdose. The State sought review, urging either (1) adoption of a “substantial-factor” test in such cases or (2) clarification of but-for causation.

2. Summary of the Opinion

The Supreme Court of Ohio reversed the court of appeals and reinstated the trial court’s judgment. The Court held that, even assuming R.C. 2903.04(A) and R.C. 2925.02(A)(3) require but-for causation, the State need prove only that the death or serious physical harm would not have occurred absent the defendant’s conduct. The State need not prove the defendant’s conduct alone would have caused the harm, and the presence of other necessary causes does not defeat but-for causation.

The Court declined to adopt (or reject) a substantial-factor test because the evidence—viewed in the State’s favor—was sufficient to permit a rational factfinder to find but-for causation. It also held the certified conflict with State v. Carpenter, 2019-Ohio-58 (3d Dist.) was improvidently certified.

3. Analysis

3.1. Precedents Cited

  • Burrage v. United States, 571 U.S. 204 (2014): The Court used Burrage to frame “causation” as including both actual cause and legal (proximate) cause, and to acknowledge that courts sometimes depart from strict but-for causation in complex causal settings. Seymour ultimately resolves the case within the but-for framework, but Burrage supplies the vocabulary and the caution that causation debates often arise in multi-factor harms.
  • Paroline v. United States, 572 U.S. 434 (2014): Cited for the distinction between actual and legal cause and for the broader observation that alternative causal standards may sometimes be used to avoid anomalous results where harms are jointly produced. Seymour leverages Paroline’s taxonomy while emphasizing it need not choose an alternative test here.
  • Rieger v. Giant Eagle, Inc., 2019-Ohio-3745: Cited for Ohio’s articulation of the but-for test (“whether the harm would not have occurred but for the defendant’s act or failure to act”). Seymour then clarifies how that test operates when multiple necessary causes combine to produce a death.
  • State v. Yerkey, 2022-Ohio-4298 (DeWine, J., dissenting): Cited for describing legal/proximate cause in terms of foreseeability; Seymour expressly sidelines proximate cause because the dispute is about actual cause.
  • Strother v. Hutchinson, 67 Ohio St.2d 282 (1981) and Clinger v. Duncan, 166 Ohio St. 216 (1957): Used to note the “murky boundaries” of proximate cause, again to explain why the opinion focuses on actual cause instead.
  • State v. McFarland, 2020-Ohio-3343 (plurality opinion), State v. Jenks, 61 Ohio St.3d 259 (1991), State v. Smith, 1997-Ohio-355, and State v. Pountney, 2018-Ohio-22: These cases provide the governing sufficiency-of-the-evidence framework: appellate courts ask whether any rational trier of fact could find the elements beyond a reasonable doubt when viewing the evidence in the prosecution’s favor, and they do not weigh credibility.
  • State v. Dunn, 2024-Ohio-5742, State v. Roberts, 2025-Ohio-5120, State v. Griffin, 13 Ohio App.3d 376 (1st Dist. 1979), State v. Jackson, 57 Ohio St.3d 29 (1991), and Michalic v. Cleveland Tankers, Inc., 364 U.S. 325 (1960): Cited to correct the Tenth District’s approach by reaffirming that circumstantial evidence counts in sufficiency review, has the same probative value as direct evidence, and can be more persuasive.
  • State v. Carpenter, 2019-Ohio-58 (3d Dist.): Carpenter is referenced as an example of applying a substantial-factor test to affirm convictions in a mixed-drug overdose. Seymour neither endorses nor rejects that approach; instead, it resolves the case under but-for causation and declares the inter-district “conflict” improvidently certified.

3.2. Legal Reasoning

(a) The key doctrinal move: “but-for” does not mean “sole cause.”
The Court “adopt[ed] the State’s second proposition of law” as “an accurate explanation of the application of the but-for test”: even if a statute requires but-for causation, the prosecution need only show the harm would not have occurred absent the defendant’s conduct. It need not show the defendant’s conduct alone would have caused the harm; multiple necessary causes can coexist without negating but-for causation.

(b) The evidentiary move: sufficiency review must consider circumstantial evidence, not just expert “magic words.”
The Tenth District effectively required direct expert testimony that heroin was the but-for cause. Seymour holds that this misapplies sufficiency review, which looks to all evidence (including circumstantial) and asks whether a rational factfinder could find the element proved beyond a reasonable doubt.

(c) Application to the record.
Although the pathologist could not “single out” one drug and could not say whether Adam would have lived without any one drug, the Court highlighted circumstantial and expert evidence supporting a rational inference that heroin was necessary to the fatal outcome:

  • Expert testimony that overdose death is most commonly associated with heroin among the four drugs and that heroin levels were “typical to see in a heroin-related death.”
  • Expert testimony describing Benadryl overdose deaths as “extremely rare,” Ritalin deaths as “very rare,” and kratom deaths as only “becoming more common.”
  • Lay testimony that Adam had appeared sober before and after the trip with Seymour, and died within about an hour after returning home.
  • Physical evidence at the scene (syringe, paraphernalia, heroin residue) indicating heroin use in the narrow time window before death.
  • Evidence suggesting Adam regularly used the other substances, while heroin use had been avoided for years—supporting the inference that heroin was the decisive change.

From these facts, the Court concluded that a rational trier of fact could find that, without Adam’s ingestion of the heroin Seymour furnished, the death would not have occurred—satisfying actual causation under the but-for test.

(d) Judicial restraint on “substantial factor.”
Because but-for causation “ably resolves this case,” the Court declined to address whether Ohio should adopt a substantial-factor test in mixed-drug overdose cases and deemed the State’s first proposition “improvidently accepted.”

3.3. Impact

On overdose prosecutions: Seymour strengthens the State’s ability to prove causation in mixed-drug overdose cases without an expert explicitly stating that the defendant’s drug was “the” but-for cause. Prosecutors may rely on a mosaic of evidence—drug concentrations, relative lethality, timing, scene evidence, and usage history—to support the inference that the death would not have occurred absent the defendant-supplied controlled substance.

On trial strategy and expert testimony: Medical experts may continue to testify cautiously about synergistic effects and may refuse to rank drugs as “the cause.” Seymour signals that such caution does not necessarily preclude a finding of but-for causation if the broader record permits the necessary inference.

On appellate review: The decision is also a corrective about sufficiency review: appellate courts may not discount circumstantial evidence simply because direct testimony does not use a particular causal formulation.

On doctrine: Seymour leaves open whether Ohio will adopt a substantial-factor test in any category of criminal cases. For now, the Court’s holding clarifies that traditional but-for causation can accommodate scenarios with multiple necessary contributing causes—limiting the practical need for an alternative test in many mixed-drug prosecutions.

4. Complex Concepts Simplified

  • Actual cause (but-for cause): A factual question—would the harm have happened if the defendant’s conduct had not occurred? If the answer is “no,” actual cause is satisfied.
  • Legal/proximate cause: A separate limit on liability often tied to foreseeability. Seymour does not decide proximate cause questions because the dispute was about actual cause.
  • “But-for” does not equal “sole cause”: A defendant’s act can be a but-for cause even when other factors are also required for the harm to occur, so long as the harm would not have occurred in the absence of the defendant’s act.
  • Substantial-factor test: An alternative sometimes used when multiple forces contribute to harm, potentially allowing liability even where strict but-for causation is hard to state. Seymour does not adopt it.
  • Synergistic effect: Drugs can amplify each other, so the combined effect is greater than each drug alone. That medical reality does not, by itself, defeat but-for causation under Seymour.
  • Sufficiency of the evidence: On appeal, the question is not whether judges believe the evidence, but whether—viewed in the State’s favor—it could permit any rational factfinder to convict.
  • Circumstantial vs. direct evidence: Direct evidence states a fact outright; circumstantial evidence supports an inference. Both can prove elements beyond a reasonable doubt, and circumstantial evidence may be especially powerful in causation disputes.

5. Conclusion

State v. Seymour establishes a practical and prosecution-significant clarification of but-for causation under R.C. 2903.04(A) and R.C. 2925.02(A)(3): the State need not prove the defendant’s conduct was the sole cause of death or serious physical harm; it need only prove the harm would not have occurred absent that conduct, even where other necessary causes exist. The decision also reinforces that sufficiency review must account for the full evidentiary record, including circumstantial proof, and cannot demand direct expert testimony in a particular verbal formula.