Alcohol and cannabis are the two most widely used substances of abuse in the world and are the psychoactive substances most often found in seriously and fatally injured drivers. In a recent study, it was observed that individuals who reported both driving under the influence of alcohol (DUIA) and the influence of cannabis (DUIC) experienced collision risk that was nearly 4 times that of individuals who reported driving after using only one of these drugs. Recent research in the United States and Canada indicates that the prevalence of DUIC among young drivers of high school and university age, and young adults is similar to, or higher than, the prevalence of DUIA. This is a serious public health issue, since motor vehicle collisions are the leading cause of death in this age group. Given the frequency with which alcohol and cannabis are consumed together, it is important to understand their combined effects on driver behaviour. The current study will examine the acute effects of a moderate dose of cannabis (12.5% THC) combined with an intoxicating amount of alcohol (BAC=0.08) on driving simulator performance of young drivers. Following an eligibility screening and practice session, a total of 70 participants aged 19 to 29 years will each complete 4 experimental sessions. During each session, participants will drink alcohol or placebo alcohol and smoke an active or placebo cannabis cigarette. The effects of alcohol and cannabis on the performance of driving-related skills will be assessed using a high-fidelity driving simulator. Cognitive, psychomotor, and mood effects will also be assessed.

The proposed study will pursue the following primary aims:

Aim 1: Examine the acute effects of a moderate dose of cannabis (12.5% THC) combined with an intoxicating amount of alcohol (BAC=0.08) on driving simulator performance of young drivers. Simulated driving performance, tests of cognition, verbal memory, and mood will be measured concurrently with BAC and levels of cannabinoids in biological fluids before and after acute drug exposure in male and female drivers aged 19 to 29. BAC and biological fluids will be measured up to 5 hours following drug exposure.

Aim 2: Explore the effects of driving history, driving attitudes, and individual difference measures (e.g., demographics, drug and alcohol use, etc.) on the acute effects of alcohol and cannabis on driving simulator performance of young drivers. Exploratory analyses will be undertaken to determine if the acute effects of cannabis plus alcohol on the driving simulator task are influenced by these measures.

Study Design and Duration

This study will be a within-subjects, double-blind, double-dummy, placebo-controlled, counterbalanced, randomized clinical trial assessing the impact of alcohol and cannabis combined on driver behaviour. Although a placebo condition is part of the study, this is not a treatment study.

Initial contact with potential participants will be made via telephone, and study personnel will conduct a telephone screen for eligibility. Upon eligibility confirmation by telephone, participants will be asked to attend CAMH for an eligibility assessment. Participants will attend CAMH for a total of 6 study sessions (an eligibility assessment, a practice day, and 4 test sessions).

At each of four test sessions, participants will undergo one of these alcohol and cannabis exposure conditions: 1) placebo alcohol and placebo cannabis; 2) intoxicating dose of alcohol and placebo cannabis; 3) placebo alcohol and active cannabis, and; 4) intoxicating dose of alcohol and active cannabis. The order of these conditions will be randomly assigned. Participants will complete the alcohol manipulation followed by the cannabis manipulation. The alcohol and cannabis exposure sessions will be separated by at least 72 hours.

Participants will be asked not to use cannabis for 72 hours and alcohol for 48 hours prior to attending CAMH.

In certain instances, the Qualified Investigator may ask a participant to return for re-screening, e.g. repeat of urine test or other assessments performed for eligibility assessment. Also, in case of unforeseen delays in scheduling study participation, the Qualified Investigator will determine if there is a need to ask a participant to repeat some assessments, e.g., physical examination.

Inclusion Criteria:

• Use of cannabis at least once a week confirmed by urine point-of-care testing;
• Males who report consuming at least 5 drinks and females who report consuming at least 4 drinks in about 2 hours in the past 6 months and at least one episode of rapid alcohol consumption in the past 6 months (3 or more drinks over a span of one hour)
• 19-29 years of age;
• Holds a class G or G2 Ontario driver's licence (or equivalent from another jurisdiction) for at least 12 months;
• Willing to abstain from using alcohol for 48 hours and cannabis for 72 hours prior to Practice and Test Sessions.
• Willing to abstain from all other drugs not prescribed for medical purposes for the duration of the study;
• Provides written and informed consent.

Exclusion Criteria:

• Urine toxicology screens negative for cannabis upon eligibility assessment;
• Diagnosis of severe medical or psychiatric conditions;
• Females: Pregnancy or breastfeeding;
• Meets criteria for Alcohol or Substance Dependence (current or lifetime) (DSM-IV);
• Is a regular user of medications that affect brain function (i.e., antidepressants, benzodiazepines, stimulants);
• Taking medications or have any medical condition for which alcohol is contraindicated;
• First-degree relative diagnosed with schizophrenia;
• Severe allergy to citrus (lemon-lime).