Field sobriety tests were poor at detecting cannabis impairment in heavy users | Research

The takeaway

Standard Field Sobriety Tests were only mildly sensitive to cannabis impairment in heavy users, while a specific oral fluid device (Drager) showed high sensitivity for detecting THC.

Read this if you want to understand the challenges of detecting cannabis-impaired drivers.

Standard field sobriety tests were only mildly sensitive to cannabis

What the researchers found

Twenty heavy cannabis users participated in a placebo-controlled study where they smoked cannabis (400 micrograms/kg THC) with or without alcohol. Standard Field Sobriety Tests (SFST) were only mildly sensitive to cannabis-induced impairment.

Cannabis alone significantly affected performance on the one-leg stand test (p=0.037), but the overall SFST battery was poor at detecting impairment. The combination of cannabis and alcohol affected horizontal gaze nystagmus (p=0.029), but this test is primarily validated for alcohol.

For roadside drug detection, the Drager Drug Test 5000 demonstrated high sensitivity for detecting THC in oral fluid, while the Securetec Drugwipe 5 had low sensitivity. The results suggested different tools are needed for detecting cannabis impairment versus detecting cannabis presence.

Why it matters

Field sobriety tests are the primary roadside tool for detecting impaired drivers. Their poor sensitivity to cannabis impairment means officers may not identify cannabis-impaired drivers, creating a gap in road safety enforcement.

The numbers in context

20 heavy cannabis users. THC dose: 400 micrograms/kg. Cannabis significantly affected one-leg stand (p=0.037). Cannabis + alcohol affected horizontal gaze nystagmus (p=0.029). Drager 5000: high THC sensitivity. Drugwipe 5: low sensitivity.

How the study worked

Double-blind, placebo-controlled study with 20 heavy cannabis users (15 males, 5 females, mean age 24.3). Participants received THC (400 micrograms/kg) with alcohol (targeting BAC 0.5 or 0.7 mg/mL) or placebo combinations. SFST performance and oral fluid THC detection were assessed.

What this study cannot tell us

Heavy users were tested, and tolerance likely reduced observable impairment. Time between cannabis use and testing may have affected results. The study could not determine whether the lack of SFST sensitivity reflected tolerance or genuine limitations of the tests for detecting cannabis effects.

How to read the evidence

Placebo-controlled study with relevant real-world application. Moderate sample size. Heavy user population may limit generalizability to occasional users.

When this study was published

Published in 2012. Cannabis-impaired driving detection remains a significant challenge. Newer devices and approaches have been developed but no gold standard exists.

The bigger picture

As cannabis legalization expanded, the inability of standard field sobriety tests to reliably detect cannabis impairment became a significant law enforcement and public safety challenge. Oral fluid testing offered a detection approach but detects presence, not necessarily impairment.

Questions still open

Should new field sobriety tests be developed specifically for cannabis? Does tolerance explain the poor SFST sensitivity, or are the tests inherently insensitive to cannabis effects? Can oral fluid THC levels reliably indicate impairment level?

Common questions

Can police tell if you are high from a field sobriety test?

This study found standard field sobriety tests were poor at detecting cannabis impairment in heavy users. These tests were designed and validated for alcohol, and cannabis produces different types of impairment that the tests are not well-suited to detect.

Can a test detect THC in saliva?

Yes, but detection and impairment are different things. The Drager Drug Test 5000 reliably detected THC in oral fluid, but having THC in saliva does not necessarily mean a person is impaired, especially in regular users who may have residual THC.

Read the original research

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, 223(4), 439-46

View on PubMed ↗

Citation

Bosker, W M; Theunissen, E L; Conen, S; Kuypers, K P C; Jeffery, W K; Walls, H C; Kauert, G F; Toennes, S W; Moeller, M R; Ramaekers, J G. (2012). 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, 223(4), 439-46.