Part 1: How Police Test for Cannabis Impairment: A Three-Part Series

Part 1: How Police Test for Cannabis Impairment—The Methods and Rationale

The legalization of cannabis in many states has created a complex challenge for law enforcement—how to determine 

if a driver is under the influence of cannabis. Unlike alcohol, cannabis impairment is harder to quantify, as there 

is no universally accepted standard. In this first part of the series, we examine the common methods police use to 

detect cannabis impairment and the rationale behind them.

Field Sobriety Tests (FSTs):

- Horizontal Gaze Nystagmus Test: Detects involuntary jerking of the eyes. Reliable for alcohol but not specific to cannabis.

- Walk-and-Turn Test: Tests the driver’s ability to follow instructions and maintain balance.

- One-Leg Stand Test: Assesses balance and focus by asking the driver to stand on one leg.

Saliva Swab Testing:

- Oral fluid tests analyze THC levels in saliva, providing quick results.

Blood Tests:

- Measures the concentration of THC in the bloodstream. However, THC levels do not correlate directly with impairment.

Drug Recognition Experts (DREs):

- DREs perform a 12-step evaluation to determine drug impairment.

Emerging THC Breathalyzers:

- Designed to detect recent cannabis use but still in early development stages.