Legal Limit for THC While Driving in Arizona? ARS 28-1381

There is none, and the absence is the whole story. Arizona’s DUI statute, A.R.S. § 28-1381, contains a numeric standard for alcohol, 0.08 within two hours of driving, and no numeric standard for THC or any other drug. Instead it contains two separate rules that each reach marijuana without a cutoff. One asks whether you were impaired at all. The other asks only whether an active form of the drug was in your body. A driver can violate the second rule with a blood level that no toxicologist would call impairing.

Recreational legalization did not add a limit. A.R.S. § 36-2851, part of the 2020 Smart and Safe Arizona Act, states that the Act does not allow driving while impaired to even the slightest degree by marijuana and does not prevent the State from penalizing it. The charge and its consequences are explained on our Arizona drug DUI defense page.

What Are the Two Marijuana DUI Charges in Arizona?

The impairment charge, § 28-1381(A)(1), makes it unlawful to drive or be in actual physical control of a vehicle while under the influence of any drug “if the person is impaired to the slightest degree.” The State proves it with the officer’s observations, the field sobriety tests and a drug recognition evaluation, and uses the blood result as corroboration. There is no number to meet because the question is your condition, not your concentration.

The drug-in-body charge, § 28-1381(A)(3), makes it unlawful to drive “while there is any drug defined in section 13-3401 or its metabolite in the person’s body.” Marijuana and cannabis are listed in § 13-3401. This is a per se offense with the threshold set at any detectable amount. The State does not have to prove you were impaired, only that the lab found the drug. Prosecutors charge both counts in nearly every marijuana DUI, and the (A)(3) count is the one that convicts drivers who were not impaired.

Which Marijuana Metabolites Count Under Arizona Law?

Only the ones that can impair. When THC is absorbed, the body converts it first to 11-hydroxy-THC, which is psychoactive, and then to carboxy-THC, which is not and which lingers in fat tissue for days or weeks after use. Read literally, “its metabolite” in § 28-1381(A)(3) would have made every regular user a DUI defendant every time they drove. The Arizona Supreme Court rejected that reading in State ex rel. Montgomery v. Harris, 234 Ariz. 343 (2014), holding that the statute reaches only metabolites capable of causing impairment and that a driver cannot be convicted under (A)(3) based merely on the presence of carboxy-THC.

The practical rule after Harris: a blood result showing delta-9 THC or hydroxy-THC supports the per se charge; a result showing only carboxy-THC does not. The lab report lists all three, and the first thing we check is which ones appear and at what level.

Does a Medical Marijuana Card Change the Limit?

It creates a defense, not an exemption. The Arizona Medical Marijuana Act, at A.R.S. § 36-2802(D), says a registered qualifying patient “shall not be considered to be under the influence of marijuana solely because of the presence of metabolites or components of marijuana that appear in insufficient concentration to cause impairment.” In Dobson v. McClennen, 238 Ariz. 389 (2015), the Arizona Supreme Court held that this language does not immunize cardholders from an (A)(3) prosecution, but it does give them an affirmative defense: a patient who proves that the marijuana or its metabolite was in a concentration insufficient to cause impairment is not guilty of the per se charge.

The burden is on the patient, which means the defense needs evidence, usually a toxicologist’s testimony tying the blood level to the absence of impairment. The defendants in Dobson lost because they offered none. The prescription-drug defense in § 28-1381(D) does not help, because marijuana cannot be “prescribed” by a practitioner under federal law; the card is the only route. Cardholders should also read our post on medical marijuana cards and firearms in Arizona, since a DUI conviction compounds that problem.

How Do Other States Handle THC Limits?

Three ways, which is why drivers arriving in Arizona are often surprised. Some states set a per se nanogram limit, most commonly five nanograms of delta-9 THC per milliliter of blood, above which a driver is presumed impaired or guilty. Some states use the same figure only as a permissible inference the jury may accept or reject. Arizona belongs to the third group, zero-tolerance states, where any active THC supports a per se charge. There is no federal standard, and the scientific literature has not produced a THC concentration that predicts impairment the way 0.08 does for alcohol, which is the main argument against numeric limits and the main reason Arizona has never adopted one.

What Do the Blood Numbers Mean in an Arizona Case?

Less than prosecutors suggest. Because there is no limit, a result of two nanograms and a result of twenty are charged the same way under (A)(3). The number matters in three other places. On the impairment count, the State’s toxicologist will try to tie a higher level to the signs the officer recorded, and a defense toxicologist will explain why THC levels do not track impairment the way alcohol does, especially in regular users who carry a baseline. On the cardholder defense, the number is the evidence that the concentration was insufficient to cause impairment. And in every case, the lab’s measurement uncertainty, the delay between the stop and the draw, and the sample’s storage decide whether the reported number can be trusted at all. How the sample is collected and what the lab reports is explained in our article on how police test for weed in an Arizona DUI.

What Are the Penalties for a THC DUI in Arizona?

The same as an alcohol DUI at the same level. A first offense under § 28-1381 is a class 1 misdemeanor carrying at least ten consecutive days in jail, a fine of at least $250 plus assessments, possible community restitution, and MVD consequences under § 28-1381(I). A third offense within 84 months, a DUI on a suspended license, or a DUI with a child under 15 in the car becomes felony aggravated DUI under § 28-1383. The extreme and super extreme tiers do not apply, because those are defined by alcohol concentration. Our guide to a first drug DUI in Arizona covers the sentence in detail.

How Tamou Law Group Defends a THC DUI With No Legal Limit

Pre-charge advocacy. The blood result arrives weeks after the stop. We get the lab’s complete data package, not the summary, and we preserve the body-camera footage and dispatch timeline before the charging decision. Our Phoenix DUI lawyers start that work the day you call.

Disclosure strategy. We demand the chromatograms, calibration, uncertainty and chain-of-custody records for the blood, the DRE’s face sheet and rolling log, and the officer’s training records. If the result shows only carboxy-THC, we move to dismiss the (A)(3) count under Harris. If you hold a medical marijuana card, we retain the toxicologist whose testimony the Dobson defense requires.

Plea and collateral analysis. Where a plea is the right outcome, we negotiate it with the suppression motion and the toxicology report in hand, and we explain the MVD suspension, interlock, insurance and record consequences before anything is signed. Cardholders also get a review of what a conviction means for their registration. See the Phoenix marijuana lawyer page for the possession side of Arizona cannabis law.