Police in the United States have no roadside device that measures cannabis impairment the way a breathalyzer measures alcohol. Officers use three tools: field sobriety tests, a Drug Recognition Expert evaluation, and in some states an oral fluid swab. A blood draw at a station or hospital confirms THC later.
how long after smoking can you drive
What Happens at a Traffic Stop
An officer who suspects impairment asks the driver to step out of the car. The officer then runs the Standardized Field Sobriety Test battery. NHTSA developed this three-test set for alcohol in the 1970s. The tests are Horizontal Gaze Nystagmus, Walk and Turn, and One Leg Stand.
Medical Marijuana Patients Driving Laws: A Comprehensive Guide
HGN tracks involuntary eye jerking. NHTSA states that HGN is not a reliable sign of impairment by drugs other than alcohol. Cannabis does not produce the same eye movement that alcohol does, so this test adds little evidence in a cannabis case.
Can You Drive With CBD Oil? A Buyer's Guide to Choosing a Low-Risk Product
Oral Fluid Tests
Devices such as the Abbott SoToxa and the Dräger DrugTest 5000 collect saliva. A swab goes into the mouth, and results come back in about 5 to 10 minutes. The swab reads THC, the parent compound. Blood and urine tests target THC-COOH, a metabolite.
how long after smoking can you drive
THC shows up in saliva fast. Levels peak within minutes of smoking and then drop. Most people test positive for 1 to 24 hours after use. Heavy users can test positive for longer.
Accuracy is a known problem. Oral fluid tests can react to other cannabinoids, including CBD products. Several state police agencies ran pilot programs before expanding use. The Michigan State Police tested the SoToxa in five counties in 2017 and 2018.
Blood Draws and Per Se Limits
A blood test measures THC and its metabolites. Several states set a numeric limit at which THC in blood counts as evidence of driving under the influence. Cutoffs range from any detectable amount to 5 ng/mL. Colorado and Montana use 5 ng/mL. Pennsylvania treats any detectable THC in blood as a violation. Nevada repealed its 2 ng/mL limit in 2021. The Washington Supreme Court struck down that state's per se law in 2020.
Blood THC does not track impairment well. THC leaves the blood within hours, while impairment fades on a different timeline. A positive blood test shows use. It does not show how impaired a driver was at the wheel.
Drug Recognition Experts
A Drug Recognition Expert is an officer who completed a 12-step evaluation course run by the International Association of Chiefs of Police. The steps include a divided attention test, vital signs, eye checks, and a toxicology sample. DRE opinions appear in court as evidence.
Refusing a Test
Field sobriety tests are voluntary in most states. Refusing them can be used as evidence in court, but it carries no license penalty in many jurisdictions. Chemical tests after arrest fall under implied consent laws. Refusing a blood or breath test after a lawful arrest can suspend a driver's license for 6 to 12 months, depending on the state.
What the Tests Cannot Do
- No device on the market gives a roadside impairment number for cannabis.
- Saliva and blood tests show presence of THC, not degree of impairment.
- Regular users can test positive days after last use.
- Test cutoffs and legal limits vary by state and change between legislative sessions.
Breath Tests for THC
Companies are developing breath tests for THC. None are in broad police use. A breath sample also carries a limit: THC in breath appears for a short window after smoking, so a negative reading does not rule out recent use.
If You Are Stopped
State law decides what an officer can require. A driver should know the rules in their own state before a stop happens. Penalties for a cannabis DUI match alcohol DUI penalties in most states: fines, license suspension, and possible jail time.