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A Positive Drug Test Is Not Evidence of Impairment. Here Is Why That Distinction Determines Liability.

In substance-related injury litigation, one of the most consequential errors attorneys and adjusters make is treating toxicological presence as equivalent to functional impairment. They are not the same. The difference between them frequently determines whether a case settles at nuisance value or goes to seven figures — and courts are increasingly making this distinction explicit.

When a tox screen returns positive for cannabis, it establishes one thing: that the individual used cannabis at some point within the detection window — which, depending on the compound and the individual's pattern of use, may range from hours to weeks. It establishes nothing about whether the person was impaired at the time of the incident.

What the Courts Have Said

This is not merely a clinical argument — it is a legal one with real precedent. In Rose v. Berry Plastics Corp. (Oklahoma, 2019), a court reinstated workers' compensation benefits for a claimant who tested positive for marijuana after a workplace hand injury. The court explicitly rejected the inference that the mere presence of marijuana in a person's bloodstream means they were intoxicated at the time of the incident. The claimant had smoked marijuana the evening before, not on the day of the accident — and no supervisor observed any signs of impairment. Presence was established. Impairment was not. The distinction determined the outcome. (National Law Review)

A similar dynamic has played out in Texas. In Bituminous Fire & Marine Insurance v. Ricardo Ruel, a worker sustained electrical burns and tested positive for cocaine — and admitted to using it days before the accident. Yet the jury concluded he was not impaired at the time of the incident, and the appeals court affirmed. As the court noted, unlike alcohol, there is no statutory standard that establishes per se impairment from a controlled substance. The insurer's attempt to deny benefits on the basis of the positive test alone failed because toxicological presence and functional impairment are legally and clinically distinct. (Risk & Insurance)

Why Detection Windows Are Not Impairment Windows

THC produces acute cognitive and psychomotor impairment that typically peaks within 30–90 minutes of use and resolves substantially within 3–4 hours in most users. Yet THC metabolites remain detectable in urine for days in casual users and weeks in chronic users, long after any meaningful functional impairment has resolved.

This means a worker who tests positive after a workplace accident may have last used cannabis four days prior, with zero residual impairment at the time of the incident. Or they may have used hours before. The tox screen alone cannot answer which. Only a careful clinical analysis of use pattern, timing, route of administration, tolerance, and the specific cognitive demands of the task can begin to answer that question with defensible precision.

The Same Logic Applies Across Compounds

Alcohol metabolizes predictably — approximately 0.015% BAC per hour — making impairment reconstruction tractable when timing is established. Prescription opioids present different challenges: therapeutic use, tolerance, and the distinction between physical dependence and functional impairment require clinical judgment that toxicology alone cannot provide. Emerging compounds — including therapeutic psychedelics now appearing in workplace and injury contexts — present evaluation challenges that very few forensic practitioners have the training to address.

What This Means for Your Matter

Courts have made clear that a positive drug test is the beginning of the analysis, not the end. Whether you are defending against an impairment claim or building one, the critical question is not what the tox screen showed — it is whether actual functional impairment existed at the time that matters, and whether the clinical record and pharmacological evidence support that conclusion. That assessment, made early by a qualified physician, changes everything about how you approach the matter.