Weed, THC, Car Crashes, and Fatalities

Sometimes marijuana can kill, not by overdose, but when people drive under the influence of THC. Recent research analyzed coroner records in Montgomery County, Ohio, from January 2019 through September 2024 to identify THC, the psychoactive drug derived from cannabis, in drivers in fatal crashes. Shockingly, close to half (42%) of the drivers killed in crashes tested positive for THC in the blood.

In the cases of 246 deceased drivers, each had their blood tested for THC as part of the autopsy/toxicology process. Because these are coroner-drawn blood samples (typically within hours of death), the THC levels reflected usage close in time to the crash (recent ingestion). The most important result was the high prevalence of THC positivity in fatal crashes; 103 out of 246 drivers (41.9%) tested positive for active THC. The researchers also found that the THC concentrations in blood were very high. Among the THC-positive cases, the average concentration was 30.7 ng/mL, a blood level well above impairment thresholds, often 2- 5 ng/mL.

This information was presented at the recent Clinical Congress of the American College of Surgeons. Caution is warranted until the findings are peer-reviewed and published.

The cohort consisted of drivers who died in crashes, but information was not provided on drivers in general, nonfatal crashes, or those who survived. Also, although many deceased drivers had THC in their blood, that doesn’t prove that THC caused or contributed to the crash. There may have been other substances present in the blood, like alcohol, cocaine, opioids, or benzodiazepines. Also, the study did not utilize control groups (like drivers in nonfatal crashes or non-crash drivers). Still, the findings are concerning.

According to Dr. Kevin Sabet, founder of Smart Approaches to Marijuana, “The report from Ohio is tragic but not surprising. We already know marijuana and driving do not mix, but seeing a report that shows 42% of road deaths are linked to drivers who tested positive for high levels of THC is shocking. Results like these are expected when considering how Americans generally view drugged driving. The AAA Foundation for Traffic Safety found 53% of marijuana users consume marijuana an hour or less before driving.”

Corroborating Studies

The new data are a stark reminder that concern for drugged driving—under the influence of cannabis—is not new. Nora Volkow, director of the National Institute on Drug Abuse, stated in 2014 testimony to Congress, “The most pronounced immediate threat from marijuana use is through impacting driving ability. Marijuana significantly impairs coordination and reaction time.”

A National Highway Traffic Safety Administration (NHTSA) report stated, "According to studies, marijuana can slow your reaction time, impair judgment of distance, and decrease coordination, which are essential when you are behind the wheel of a vehicle." Another NHTSA report examined toxicology results from U.S. crash victims and found that approximately 19% of tested drivers had THC in their systems. The report emphasized that cannabis was among the most commonly-detected psychoactive substances, second only to alcohol.

A 2024 study by Chen and colleagues of more than 14,000 fatally injured drivers found that one-third (33%) tested positive for cannabis. Controlled lab and simulator trials demonstrated objective impairment, even when users believed themselves safe to drive.

The U.S. findings on driving under the influence of cannabis are similar to those in European studies. A landmark study using 10,748 fatal crash drivers demonstrated a clear dose-response between blood THC and crash responsibility. Updated French national data confirmed independent and additive driving risks for THC and alcohol, with an adjusted odds ratio of 1.65 for THC alone. The odds of the outcome occurring are 1.65 times higher for a person using THC alone than for a person not using THC.

A Worsening Problem

The problem is increasing. An analysis from Boston University, drawing on federal transportation data from 2000 to 2018, found that the percentage of crash deaths involving cannabis more than doubled—from 9% in 2000 to 21.5% in 2018. The authors concluded that cannabis involvement in fatal crashes has been steadily increasing across the U.S.

A study published in JAMA Network Open in April 2025 evaluated a sample of injured (not necessarily deceased) drivers. In this group, 16.3% tested positive for THC, slightly higher than the proportion positive for alcohol (16.1%). Overall, 54.9% of all injured drivers tested positive for at least one impairing substance. Although the study included nonfatal injuries, the findings reinforce that cannabis and other drug use among drivers is common and needs to be considered in crashes.

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The public appears to believe that driving under the influence of alcohol is dangerous, but it has not extended the same logical thinking to cannabis or THC. Yet data suggests otherwise.

Although studies cannot prove that cannabis causes crashes—since detection does not necessarily equal impairment—evidence strongly supports that recent cannabis use is common among fatally injured drivers. THC impedes driving, as it acutely impairs coordination, visual function, and attention for several hours after consumption.

According to Dr. Robert DuPont, "Marijuana is the most widely detected drug among impaired, injured, and deceased drivers, second only to alcohol. Because there is no clear level at which most people are impaired by marijuana, as there is with alcohol, the best solution is to set the illegal per se limit at zero or near-zero for THC."

Marijuana use is increasing, particularly in areas with legal recreational use. Many more crashes involve mixed substance use. How much more risk accrues from combined exposures?

We need noninvasive methods of cannabis /THC testing, as with alcohol; testing when driving is impaired, after accidents. We also need more educational and prevention campaigns about driving under the influence of drugs.