How Do Cops Test for Weed in a DUI? Arizona Process, ARS 28-1381

In layers, because there is no single roadside instrument for marijuana the way there is for alcohol. An Arizona officer who suspects cannabis builds the case from what they see and smell at the window, what you say, how you perform on the standardized field sobriety tests, what a Drug Recognition Expert records in a twelve-step evaluation at the station, and finally what a crime lab finds in your blood. Each layer is designed to support the next, and each one can be challenged.

The reason the process looks this way is the statute. A.R.S. § 28-1381(A)(1) makes it a DUI to drive while impaired to the slightest degree by any drug, which the State proves with the officer’s and the DRE’s observations. § 28-1381(A)(3) makes it a DUI to drive with a listed drug or its impairing metabolite in your body, which the State proves with the blood test. Officers collect evidence for both counts at once. Our Arizona drug DUI defense page explains the two charges and their penalties.

What Do Officers Look For at the Stop?

The report will begin with the driving: weaving, slow reaction to a light, drifting speed, a wide turn. Then the contact: the odor of burnt or raw marijuana, red or bloodshot eyes, eyelid tremors, a green coating on the tongue, slowed speech, and any paraphernalia or product in view. Then the admissions. “I smoked earlier today” or “it was last night” is the single most damaging thing an officer can write down, because it supplies the element of recent use that the physical signs only suggest. You are not required to answer questions about drug use, and a clean breath result does not help you if you have already described when you last used.

Do Field Sobriety Tests Work for Marijuana?

They were not designed for it. The three standardized tests, horizontal gaze nystagmus, walk-and-turn and one-leg stand, were validated by NHTSA to predict blood alcohol concentration, not THC. Officers give them in marijuana stops anyway and score the same clues. The nystagmus test is the clearest example of the mismatch: the involuntary eye jerking it detects is a sign of alcohol and depressants, and the drug recognition matrix itself lists nystagmus as absent for cannabis. An officer who writes that you showed six of six nystagmus clues after smoking marijuana has either observed something cannabis does not cause or scored the test wrong.

The balance and divided-attention tests are more defensible for the State, but they measure coordination, nerves, footwear, road surface and fatigue as much as impairment. The instructions must be given word for word and demonstrated, the surface must be level and lit, and the scoring must follow the manual. Body-camera footage of the test is the first thing we request, because the report and the video frequently disagree.

What Is a Drug Recognition Expert Evaluation?

A DRE is a police officer certified under the Drug Evaluation and Classification Program, run nationally by the International Association of Chiefs of Police with NHTSA. When the arresting officer suspects drugs rather than alcohol, a DRE conducts a twelve-step evaluation, usually at the station, to form an opinion about which category of drug is causing impairment. The steps are:

1. A breath alcohol test, to rule alcohol in or out
2. An interview of the arresting officer
3. A preliminary examination and first pulse
4. Eye examinations: horizontal and vertical gaze nystagmus and lack of convergence
5. Divided-attention tests: modified Romberg balance, walk-and-turn, one-leg stand, finger-to-nose
6. Vital signs and second pulse: blood pressure, temperature
7. Dark-room pupil measurements under three lighting conditions
8. Muscle tone
9. Check for injection sites and third pulse
10. Statements and interrogation
11. The DRE’s opinion
12. Toxicology: the blood sample

For cannabis, the DRE matrix expects lack of convergence, dilated pupils and an elevated pulse and blood pressure, with no nystagmus and normal muscle tone. The evaluation is an opinion built on a checklist, not a chemical measurement, and the opinion is only as good as the technique. We review the face sheet and the DRE’s rolling log, compare the pulse and pupil readings to the normal ranges on the DRE’s own chart, and look for steps skipped or run out of order.

How Does the Blood Test Work?

Under Arizona’s implied consent law, A.R.S. § 28-1321, a driver arrested for DUI has consented to a test of blood, breath, urine or other bodily substance. For marijuana the test is blood. If you refuse, the MVD imposes a separate suspension and the officer applies for a telephonic search warrant, which in Maricopa County is typically granted within minutes. The draw must be performed by a physician, registered nurse or other qualified person under A.R.S. § 28-1388; many agencies use officers trained as phlebotomists.

The sample goes to a crime laboratory, which reports delta-9 THC, 11-hydroxy-THC and carboxy-THC in nanograms per milliliter. Only the first two can support the drug-in-body charge: the Arizona Supreme Court held in State ex rel. Montgomery v. Harris (2014) that carboxy-THC, the inactive metabolite that lingers for days or weeks, cannot. What those numbers mean, and why Arizona has no numeric cutoff, is the subject of our article on the legal limit for THC while driving in Arizona.

Do Arizona Police Use Saliva Tests for Weed?

Rarely, and never as the evidence that convicts. Some agencies have experimented with roadside oral-fluid swabs that screen for several drug classes. A swab is a screening tool: a positive result gives the officer a reason to proceed to a blood draw, not a measurement a prosecutor can present as proof of concentration. The implied consent statute’s reference to “other bodily substance” would cover saliva, but in practice the chemical evidence in an Arizona marijuana DUI is blood. If a swab was used in your stop, its result, the device and the officer’s training with it become part of the defense review.

Where Does Marijuana DUI Testing Go Wrong?

In predictable places. The stop itself may lack reasonable suspicion. The odor of marijuana, which has been legal for adults to possess since 2020, is a weaker basis for extending a stop than it was before. Field sobriety tests get given on a slope, in the dark, in sandals, with instructions paraphrased. The DRE skips a step or records a pulse within the normal range and calls it elevated. The blood draw happens two hours after the stop, long enough for active THC to fall or, after certain patterns of use, to be unrepresentative of the level while driving. The vial sits in a hot patrol car. The lab’s uncertainty figure swallows the margin the prosecutor needs.

How Tamou Law Group Defends a Marijuana DUI

Pre-charge advocacy. The blood result takes weeks. We use that time to lock down the body-camera footage, dispatch logs and warrant affidavit, and to request the lab’s complete data package before the one-page report is all anyone has seen. Our Phoenix DUI lawyers start that work the day you call.

Disclosure strategy. We demand the DRE’s face sheet and rolling log, the officer’s SFST and DRE training records, the oral-fluid device records if one was used, and the chromatograms, calibration and chain-of-custody log for the blood sample. Marijuana DUI cases are lost by the State in those records far more often than at the roadside.

Plea and collateral analysis. If the blood shows only carboxy-THC, we move to dismiss the (A)(3) count under Harris. If you hold a medical marijuana card, we build the affirmative defense recognized in Dobson v. McClennen. Where a plea is the right outcome, we negotiate it with the suppression motion and a toxicology report on the table, and we explain the MVD, interlock and record consequences before anything is signed. What a first conviction involves is covered in our guide to a first drug DUI in Arizona.