When police test for weed, they work through layers: what an officer sees and hears during a stop, standardized field sobriety tests at the roadside, a Drug Recognition Expert evaluation at the station, and chemical analysis of blood, saliva, or sometimes urine. No single test settles the question the way a breathalyzer does for alcohol, because THC levels in the body don’t line up neatly with how impaired someone actually is. That gap between detection and impairment shapes every method officers use and every charge that follows.
Research from the University of Sydney found that higher blood THC concentrations were only weakly associated with impairment in occasional users, and no significant relationship at all in regular users. A daily user can test at a level that would floor a first-time user and still drive normally. That is the core reason officers stack methods rather than relying on one number.
What Officers Look For at the Roadside
The process usually starts with observation. Officers note bloodshot or glassy eyes, the smell of cannabis, slowed or confused speech, and problems with coordination. Those signs don’t prove impairment on their own, but they build the reasonable suspicion needed to keep testing.
If the officer suspects you’re impaired, the next step is typically Standardized Field Sobriety Tests. NHTSA recognizes three: the Horizontal Gaze Nystagmus test, the Walk-and-Turn, and the One-Leg Stand.1NHTSA. DWI Detection and Standardized Field Sobriety Test (SFST) Participant Manual These tests were designed for alcohol. HGN looks for involuntary eye jerking as you track a stimulus; alcohol reliably causes it, cannabis generally does not in roadside settings. The Walk-and-Turn and One-Leg Stand tests measure balance, coordination, and your ability to follow multi-step instructions, all of which cannabis can affect.
The catch, and it’s a big one for anyone taking these tests: a 2023 study found that officers classified 81% of THC-exposed participants as impaired on field sobriety tests, but also classified 49% of placebo participants (who had consumed no THC at all) as impaired. The study concluded that field sobriety tests alone “may be insufficient to identify THC-specific driving impairment.”2PMC (PubMed Central). Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis That false-positive rate would be unacceptable for alcohol enforcement, and officers know it. That is why cannabis stops often escalate to a second, deeper evaluation.
The Drug Recognition Expert Evaluation
When field tests suggest drug impairment but can’t identify the drug, a Drug Recognition Expert may take over. DREs are officers trained through a national program to identify impairment across seven drug categories, cannabis included. Their evaluation follows a 12-step protocol developed by the International Association of Chiefs of Police.3International Association of Chiefs of Police. 12 Step Process
The evaluation happens in a controlled setting, usually a station. A DRE measures blood pressure, pulse, and temperature, checks pupil size under three lighting conditions with a pupilometer, examines muscle tone, and tests for lack of convergence, which is the inability of the eyes to cross when focusing on a close object. Four psychophysical tests follow: Modified Romberg Balance, Walk-and-Turn, One-Leg Stand, and Finger-to-Nose.
Cannabis produces a distinctive pattern. Pupils may be dilated or within normal range. Pulse and blood pressure are usually elevated. Lack of convergence is typically present. And cannabis generally does not cause nystagmus, which helps the DRE rule out depressants and inhalants. The DRE then forms an opinion about which drug category is involved, and that opinion is checked against a toxicology sample.
Chemical Tests and What Each One Actually Measures
Chemical testing gives objective evidence that a substance was in your body. What it shows varies sharply by sample type, and one distinction matters more than any other: delta-9-THC is the compound that actually causes impairment, while THC-COOH is the inactive metabolite your body produces afterward. Detecting the metabolite proves you used cannabis at some point. It says nothing about whether you were impaired at the time of the test.
Blood
Blood is the closest thing law enforcement has to a measure of recent impairment because it detects active delta-9-THC, not just metabolites. A blood draw is performed by medical personnel, usually at a hospital or station. Active THC drops fast after use, so the timing of the draw matters enormously. In per se states, the concentration at the moment of the draw is the number that gets compared to the legal threshold.
Oral Fluid (Saliva)
Saliva tests are becoming more common as a roadside screening tool because they’re non-invasive, quick, and detect recent use. An officer swabs the inside of your cheek. THC appears in oral fluid within minutes of smoking, so a positive result points to recent consumption. Several states have piloted or adopted oral fluid programs for roadside use.
Urine
Urine tests detect THC-COOH, the inactive metabolite, not active THC. That makes them useful for confirming use but nearly useless for proving impairment at a specific time. In DUI investigations, a urine result typically serves as corroboration alongside a DRE evaluation rather than standalone proof.
Hair testing exists but doesn’t come up in traffic stops. It reflects historical use over roughly 90 days and has no value for proving impairment at any given moment. You’re more likely to see it in probation monitoring or employment screening.
How Long THC Stays Detectable
Detection windows depend on the test, how often you use, your metabolism, and the potency of what you consumed. These are general ranges.
- Blood: active THC falls below detection for most occasional users within a few hours of a single use. One study found THC undetectable in most participants by 22 hours after smoking. Chronic daily users can stay positive a day or two longer because THC accumulates in fatty tissue and releases slowly.4National Center for Biotechnology Information (NCBI). Identification of Recent Cannabis Use: Whole-Blood and Plasma Free and Glucuronidated Cannabinoid Pharmacokinetics following Controlled Smoked Cannabis Administration
- Oral fluid: detectable almost immediately after smoking. A controlled study found median last-detection times of 27 hours for occasional smokers and over 30 hours for daily smokers, with wide individual variation.5PMC (PubMed Central). Oral Fluid Cannabinoid Concentrations Following Controlled Smoked Cannabis in Chronic Frequent and Occasional Smokers
- Urine: a single use is typically detectable for about 3 days. Moderate use (around four times a week) extends that to 5 to 7 days. Daily use pushes it to 10 to 15 days. Chronic heavy use can stay positive for 30 days or more as THC-COOH continues to leach from fat stores.
- Hair: up to 90 days based on a standard 1.5-inch sample. Reflects historical use, not current impairment.
What a Positive Test Actually Proves in Court
There’s no federal standard for how much THC in blood makes you legally impaired. States take three fundamentally different approaches, and which one applies to you can determine whether a positive test alone is enough to convict.
- Per se limits. Several states set a specific blood THC concentration above which you’re presumed impaired by law. Colorado, Washington, Montana, and Illinois use 5 ng/mL. Nevada and Ohio use a stricter 2 ng/mL.6Governors Highway Safety Association. Drug-Impaired Driving
- Zero tolerance. About a dozen states prohibit driving with any detectable THC or its metabolites. Arizona, Delaware, Georgia, Indiana, Iowa, Michigan, Oklahoma, Pennsylvania, Rhode Island, Utah, and Wisconsin all fall here. Actual impairment doesn’t matter. Any detectable THC is a violation.7National Conference of State Legislatures. Drugged Driving – Marijuana-Impaired Driving
- Effect-based or reasonable inference. Many states set no specific number. Prosecutors must prove impairment through officer observations, the DRE evaluation, and toxicology combined. Colorado uses a reasonable-inference framework: at or above 5 ng/mL, a jury can infer impairment, but you’re allowed to rebut it with evidence.7National Conference of State Legislatures. Drugged Driving – Marijuana-Impaired Driving
The practical difference is huge. A daily medical user in a zero-tolerance state could test positive days after last use and face charges without ever having been impaired behind the wheel. The same person in an effect-based state might have a viable defense.
Blood Draws, Warrants, and What Happens If You Refuse
Because THC levels in blood drop quickly, timing is legally critical, and so is how the draw is obtained. Two Supreme Court decisions set the constitutional lines.
In Missouri v. McNeely (2013), the Court held that natural dissipation of a substance in the bloodstream does not, by itself, justify a warrantless blood draw. Officers who can reasonably get a warrant have to get one.8Justia. Missouri v. McNeely, 569 U.S. 141 (2013) A warrantless draw is allowed only when genuine exigent circumstances exist beyond the fact of metabolism. In Birchfield v. North Dakota (2016), the Court separated breath from blood: states can require a breath test incident to a DUI arrest without a warrant, but a blood draw is a more significant intrusion that requires a warrant or valid consent.9Justia. Birchfield v. North Dakota, 579 U.S. ___ (2016)
Every state also has an implied consent law. By accepting a driver’s license, you’ve already agreed to submit to chemical testing if lawfully arrested for impaired driving. Implied consent typically covers post-arrest chemical testing at a station or hospital, not preliminary roadside screening. Refusing after arrest triggers automatic consequences regardless of whether you’re eventually convicted. License suspension is the usual penalty, generally ranging from 180 days to a year. Some states also let prosecutors tell the jury you refused.
Many jurisdictions now run “no-refusal” enforcement periods around holidays. A judge or magistrate stays available to issue electronic warrants quickly, so officers can compel a blood draw within minutes of a refusal.
A Medical Marijuana Card Won’t Save You
A valid medical marijuana prescription or card provides no defense against impaired driving charges in any state. The reasoning tracks prescription opioids or benzodiazepines: being legally authorized to possess and use a substance doesn’t mean you can legally drive while impaired by it.
In a reasonable-inference state like Colorado, a medical user can argue that despite testing above 5 ng/mL, they’d developed tolerance and weren’t actually impaired. That defense is available to any driver, not just cardholders. In a zero-tolerance state, a medical patient faces the same exposure as a recreational user: any detectable THC is a per se violation, no matter the authorization or the driver’s actual condition.
Where the THC Breathalyzer Stands
The idea of a cannabis breathalyzer that works like an alcohol one has drawn serious investment, but the technology isn’t ready for widespread use. A September 2025 workshop at the National Institute of Standards and Technology brought together at least nine device manufacturers, and the stated purpose was to “foster an open and candid discussion for a broad view of the path forward to realize meaningful cannabis breathalyzer technology.”10NIST. Building a Path Forward for Meaningful Cannabis Breathalyzer Realization That framing tells you where things stand.
The underlying problem circles back to impairment. Even a device that accurately measures THC in breath still faces the question of what breath concentration corresponds to actual impairment, and that relationship hasn’t been established. Until it is, a cannabis breathalyzer would confirm recent use without confirming whether the driver is too impaired to drive. For now, the layered approach of observations, DRE evaluations, and chemical testing is what officers rely on.