Cannabis and Alcohol and Driving: 1+1 =3?

A new study of the effects of 10 or 25 mg cannabis brownies (18% THC) and a BAC of 50 mg/100mL on driving ability and the SFSTs were determined on 15 male and 10 female drivers with a mean age of 25.6 years. The drivers were also tested under placebo conditions and a BAC of 80 mg/100mL alone. This design allowed researchers to compare the impairment caused by alcohol and cannabis both separately and together.

Blood samples were collected and THC, 11-OH-THC, and THCCOOH concentrations were measured by LC-MS/MS. The mean peak blood THC was 3.5 ng/mL for the 25 mg THC edible. This is a relatively low blood THC concentration – well within the legal limits in most countries. Yet when combined with alcohol and cannabis, the impairment was significant.

The drivers were tested on a STSIM Drive M4000 R driving simulator, and the impairment of driving ability was measured on a global drive score (GDS), where worsening driving performance was measured by an increase in the GDS. Both preliminary screening tests conducted by the police, especially the SFST, detected the 80 mg/100mL and the 25 mg THC and 50 mg/100mL conditions. This means that standard roadside tests can identify drivers impaired by the combination of alcohol and cannabis.

The authors were concerned with the real-world applications of their study. Even though the driver may have impairment equal to or greater than at a BAC of 80 mg/100mL, the drug results would show 3.5 ng/mL THC and 50 mg/100mL BAC – both under the current THC/alcohol limits in most countries. In other words, a driver could be significantly impaired by the combination of alcohol and cannabis yet still be legally under the limit for each substance individually. This is a critical gap in public safety.

One solution to this problem was addressed in the Criminal Code of Canada in response to Canada’s legalization of cannabis in 2018. The offenses and penalties for drug-impaired driving include specific thresholds for alcohol and cannabis combined.

For drivers with 50 mg/100mL alcohol + 2.5 ng/mL THC or more:

• First offence: Mandatory minimum $1,000 fine
• Second offence: Mandatory minimum 30 days imprisonment
• Third offence: Mandatory minimum 120 days imprisonment

Since the blood THC limit is from 2 to 5 ng/mL for a lesser summary offense and greater penalties for blood THC of 5 ng/mL and greater, the drivers in this study would have been charged with the lesser offense as the mean peak blood THC concentrations were 3.5 ng/mL.

However, there is a greater offense if the blood THC concentration of the driver exceeded 2.5 ng/mL and the BAC was 50 mg/100mL or greater. Effectively, the BAC limit of 80 mg/100mL is lowered to 50 mg/100mL with the co-detection of blood THC concentration. The drivers would be charged with this greater offense after consuming 25 mg THC and a BAC of 50 mg/100mL – exactly the combination tested in the study.

This demonstrates that Canadian law has recognized the synergistic danger of alcohol and cannabis and has created specific penalties to address it. But many countries, including most US states, have not yet followed suit.

The authors of the study conclude:

“In this crossover trial of healthy adults who co-used cannabis and alcohol, cannabis edibles combined with alcohol augmented driving impairment. The legal intoxication limit in most of the US (0.08% BrAC) may be too liberal if a driver has co-used cannabis and alcohol. In this era of expanding cannabis legalization, there is a pressing public health need for improved impairment detection strategies and consideration of cannabis and alcohol co-use in policies dictating access to these substances.”

The evidence is clear: alcohol and cannabis together create a dangerous synergy that impairs driving far more than either substance alone. As cannabis legalization continues to expand across North America, policymakers must consider the combined effect of alcohol and cannabis when setting legal limits. The Canadian approach – lowering the effective BAC limit to 50 mg/100mL when THC is detected – provides a model for other jurisdictions to follow. Public safety demands nothing less.

References:

Zamarripa, C.A., Lin, S., M. Klausner, et al. “Impact of Cannabis Edibles Combined with Alcohol on Driving, Field Sobriety Performance and Subjective Effects. A Within-Participant Crossover Trial”, JAMA Network Open, 9: e269842, 2026