EVIDENCE LIBRARY
How Did Alcohol Change Field Sobriety Findings in Heavy Cannabis Users?
In a small crossover study, adding alcohol to smoked THC changed some field-test findings, but overall scores were less decisive. Baseline findings, timing and the absence of an alcohol-only arm limit attribution.
Alcohol combined with THC increased some field sobriety findings in heavy cannabis users, particularly horizontal gaze nystagmus, but the study did not establish a reliable method for assigning a driver’s signs to one drug. Individual test findings and the combined score behaved differently, and the experiment included neither an alcohol-only arm nor a fully drug-free baseline.
A small study with ongoing cannabis exposure
Bosker and colleagues enrolled 20 heavy cannabis users, 15 men and five women. Eligibility required cannabis use on at least four days per week during the preceding year. Participants were allowed to continue their usual cannabis use during the study.
Each attended sessions targeting alcohol concentrations of zero, 0.5 and 0.7 mg/mL, with booster doses used to maintain the alcohol conditions. In every session, participants smoked THC at 400 micrograms per kilogram. Field testing occurred approximately two hours after smoking, rather than immediately after the dose.
The stated alcohol targets differed from concentrations during field testing. The reported means were zero, 0.37 and 0.51 mg/mL. The methods identify serum as the matrix for laboratory alcohol assessment. Those values should not be relabeled as direct whole-blood measurements without a documented conversion.
The strongest alcohol-related finding involved eye movements
The combination conditions were significantly related to horizontal gaze nystagmus findings, p = 0.029. The overall field-test score, p = 0.085, and one-leg stand, p = 0.072, showed trends that did not meet the study’s conventional 0.05 significance criterion.
The table classified 16% of participants as impaired overall at baseline, 30% after THC with alcohol placebo, 21% after THC with the lower alcohol target and 44% after THC with the higher target. Available numbers differed because of dropouts and missing baseline data. These percentages do not form a simple monotonic individual dose-response rule.
The study’s overall classification required impairment on at least two of the three tests. A positive individual test and a positive combined classification therefore answered different questions.
Baseline findings complicate interpretation
At the separate baseline assessment, 58% met the study’s walk-and-turn impairment criterion and 21% met its one-leg stand criterion, while none met its nystagmus criterion. Baseline did not represent a verified cannabis-free state: participants were frequent users with residual THC exposure.
After THC with alcohol placebo, one-leg stand findings increased significantly relative to baseline, p = 0.037. Walk-and-turn and the overall score did not significantly discriminate that condition from baseline. The authors considered both tolerance and test timing as possible explanations for limited sensitivity.
These observations do not establish that heavy users are protected from all cannabis effects, or that absence of nystagmus proves absence of alcohol. They show how population, scoring and time after use can change what a test detects.
Detection was a separate part of the experiment
The investigators also studied two oral-fluid THC screening devices. Their detection rates differed, and performance generally changed with time after smoking. Because participants were not drug-free, this study could not establish a false-positive rate from a true negative reference group.
A positive oral-fluid screen identified exposure under the device’s testing conditions; it did not measure the amount of alcohol, quantify impairment or establish which substance caused an observed sign.
Limits on a DUI opinion
The crossover design helps compare conditions within participants, but every active session included smoked THC. It cannot independently estimate alcohol’s effects in the absence of THC or prove a specific alcohol–cannabis interaction requiring a complete factorial design. The small, selected sample and delayed testing also restrict generalization.
The study did not measure driving during these field-test assessments. The site’s separate edible-THC and alcohol driving trial examines field tests alongside simulated driving in a different population and exposure setting. The trauma examination analysis illustrates why an incomplete or modified clinical examination should retain its own limitations.
Primary source
Bosker WM and colleagues. A placebo-controlled study to assess Standardized Field Sobriety Tests performance during alcohol and cannabis intoxication in heavy cannabis users and accuracy of point of collection testing devices for detecting THC in oral fluid. Psychopharmacology. 2012;223:439–446. https://doi.org/10.1007/s00213-012-2732-y.