Detecting impairment associated with cannabis with and without alcohol on the Standardized Field Sobriety Tests

Cannabis and alcohol are the most popular drugs amongst recreational users and most prevalent in injured and deceased drivers. The Standardized Field Sobriety Tests (SFST) are commonly used to establish impairment due to drugs and alcohol, but limited empirical evidence exists concerning the combined effects of these drugs on SFST performance.

The sample comprised 80 individuals (31 females; 49 males). Age ranged between 21 and 35 years (M = 26.5, SD = 5). Forty participants (15 females; 25 males) took part in the low alcohol condition (BAC, <0.05 %), and 40 participants (16 females; 24 males), took part in the high alcohol condition (BAC, >0.05 %). For each part of the study, two levels of ∆9-tetrahydrocannabinol (THC) were administered (1.8 and 3 % THC) or a matching placebo cigarette (0 % THC) in combination with alcohol. Performance on the SFST was assessed 30 min post-dosing.

A number of significant differences in SFST performance were identified with 28 % of the sample failing the test (when the head movement and jerks sign was included) when low alcohol and low THC were administered together. When a higher dose of alcohol was administered with a low dose of THC, 38 % of the sample failed the test, and 35 % also failed when the high dose of alcohol was combined with a higher dose of THC.

The current results highlight the limited ability of the SFST to identify drug consumption in the absence of any evidence of driving impairment or physiological indicators.