Neuroscience of Alcohol and Marijuana Impaired Driving

Alcohol is one of the most widely used intoxicants. The effects of driving while intoxicated are well documented, leading to the laws and regulations behind drunk driving. Marijuana is also a commonly abused drug, whose use is increasing with widespread legalization/decriminalization in many US states and use of medical marijuana. Marijuana use is linked to cognitive impairment and is likely be the cause of intoxication-induced accidents. The effects of marijuana intoxication on driving impairments are less documented than those of alcohol. However, most marijuana users also consume alcohol when smoking cannabis, and preliminary data strongly suggest that driving impairment from both drugs used together is synergistic rather than just additive.

Data are being gathered currently in regards to the risk of marijuana-impaired driving from our NIDA-funded Neuroscience of Marijuana Impaired Driving study. Previously we had a grant award from NIAAA that investigated alcohol-impaired driving using a similar design. The current proposed study combines elements of both the NIDA and NIAAA studies, to assess the cognitive and brain impairment due to the simultaneous combination of beverage alcohol and smoked marijuana.

Our own prior NIAAA-funded grant, the Brain and Alcohol Research with College Students (BARCS) study, along with epidemiological investigations reveal that most marijuana smokers also consume alcohol when intoxicated. These drugs interact pharmacodynamically and change each other's levels in the user's blood and saliva reference Marilyn study. They both have separate, deleterious effects on driving. These effects are not additive but rather multiplicative. A person using both substances will show more deleterious effects on driving performance than the same individual using just one of these substances. This study will aim to investigate the brain and behavior in the same individuals, using a similar design to the current Neuroscience of Marijuana Impaired Driving and the prior Alcohol and Driving Grant, that used similar techniques and measures to quantify drunk automobile driving. We hypothesize that alcohol and marijuana use combined will lead to greater impairment in a simulated driving task, as well as other driving-related cognitive impairments. In a randomized, counterbalanced, double-blind study, we will dose participants with alcohol to a legal level of 0.05% blood alcohol content, then we will administer a moderate inhaled dose of THC marijuana or placebo marijuana, using paced inhalation that employees a vaporizer. Participants will comprise 10 regular alcohol and marijuana consumers aged 21 to 40 years of age; all participants must report smoking marijuana and drinking alcohol together. Of the 10, 5 will be occasional marijuana smokers and 5 frequent marijuana smokers. Following this dosing, we will assess impairment through cognitive testing as well as a simulated driving test through fMRI and neuropsychological tests. Samples of breath, blood and oral fluid will also be collected at multiple time points throughout the study visits to be measured for alcohol and THC concentration and its metabolites. This allows clarification between the relationship of impairment, as well as subjective and objective intoxication, and levels of THC and its metabolites in the users system.