Effects of Cannabis/Alcohol on Driving Performance and Field Sobriety Tests

Driving under the influence of alcohol and cannabis presents significant safety risks. In the real world, impairment is usually determined via the standardized field sobriety tests (SFSTs), a series of psychophysiological tests conducted by law enforcement officers. These include the Horizontal Gaze Nystagmus (HGN; eye movements when following the officer's fingers in space), Walk and Turn (WAT; walking nine steps, turning, and walking back), and One Leg Stand (OLS; holding each leg in the air for a designated period of time).

Despite a vast literature documenting impaired driving behavior due to alcohol consumption, much less is known regarding driving under the influence of cannabis, and even less about how the combination of alcohol and cannabis may affect driving behavior. This study will examine whether either, or both, conditions significantly impact performance in a driving simulator, on cognitive measures, and during the comprehensive Drug Recognition Expert evaluations (which includes SFSTs, as well as additional measures).

Alcohol studies. Higher blood alcohol levels (BACs) have been consistently related to impaired driving. BACs greater than .08% have been associated with 5.5 times higher crash risk compared to those without alcohol or drugs. In terms of detecting alcohol-related impairment, SFSTs have consistently been shown to be predictors of a blood alcohol concentration of .08%.

Cannabis studies. Several studies suggest higher doses of whole-blood or plasma delta-9 tetrahydrocannabinol (THC) concentration are associated with increased crash risk and crash culpability. However, attempts to define a cut-off point for blood THC levels have proven to be challenging. Unlike alcohol, for which a level can be reasonably measured using a breathalyzer (and confirmed with a blood test), detection of a cut-off point for intoxication related to THC concentration has eluded scientific verification. Most recently, we did not find any correlation between blood THC concentrations and driving performance. To date, the relationship between SFSTs and cannabis use/impairment have been mixed across studies.

Cannabis and Alcohol. There have been a limited number of studies examining the combination of cannabis and alcohol on driving and/or field sobriety test performance.

With respect to subjective effects, alcohol has been found to potentiate the duration of the cannabis effects, with subjective effects being longer in the cannabis-alcohol combinations compared to either drug alone. Participants also reported more effects after alcohol and cannabis were combined, compared to alcohol alone. Another study found that alcohol pre-treatment decreased the latency to the cannabis effects, and increased the duration of the effects. It has also been hypothesized that frequent cannabis users may develop cross-tolerance to the effects of alcohol.

Some studies have found SFSTs to be only mildly sensitive to cannabis effects in heavy cannabis users, but sensitive to doses of alcohol.

There have also been suggestions that the concurrent presence of cannabis and alcohol may result in increased THC and 11-Hydroxy-Δ9-tetrahydrocannabinol (11-OH-THC) levels, possibly by affecting initial absorption (resulting in higher concentrations immediately post-inhalation; the overall area under the curve (AUC) does not differ based upon alcohol ingestion), although the possible mechanisms for this - such as increased cardiac output facilitating absorption via increased pulmonary capillary flow, or less cannabis self-titration after alcohol intake - are as yet unresolved.

Research Design

One-hundred twenty-five healthy volunteers will be randomized into one of 5 conditions; those who receive 1) low dose alcohol and placebo cannabis, 2) low dose alcohol and THC, 3) high dose alcohol and placebo cannabis, 4) placebo alcohol and THC, and 5) double placebo.

Cannabis, acquired from the National Institute on Drug Abuse Drug Supply Program, will be inhaled ad libitum and/or ingested alcohol will take place at the beginning of the day followed by the completion of driving simulations, components of the Drug Recognition Expert (DRE) evaluations, and bodily fluid draws (e.g., blood, oral fluid [OF] saliva, breath) over the subsequent 4 hours after ingestion.

Driving simulations will be conducted using a 3-monitor, fully-interactive STISIM (Systems Technology Inc. Simulator) console system. The simulations will be similar to those used in our recently completed study of 191 cannabis users.

Drug Recognition Expert (DRE) evaluations will be conducted by DREs, and will consist of 1) Preliminary Examination and First Pulse; 2) Eye Examinations: Horizontal gaze nystagmus (HGN); Vertical gaze nystagmus (VGN); Lack of convergence (LOC); 3) Divided Attention Psychophysical Tests: Modified Romberg balance; Walk and turn; One leg stand; Finger to nose; 4) Vital Signs and Second Pulse (Blood pressure, temperature, pulse); 5) Dark Room Examinations (pupillometer): Assess for dilation or constriction; pupillary response to light; 6) Examination for Muscle Tone; 7) Third Pulse.

Cannabinoids levels in blood, oral fluid, and breath will be determined via liquid chromatography with tandem mass spectrometry (LC-MS-MS).

The results of this study will further advance the understanding of the impact of acute cannabis and alcohol administration on driving performance, as well as the best methods for detecting driving under the influence of these substances.