Criminal Defense

THC / Marijuana DUI Defense in Montana

Cannabis is legal in Montana, but driving impaired is not. THC DUI cases turn on science that can be challenged, and that is where an experienced defense matters.

Written by

Misty Gaubatz, Partner, A&M Law

Licensed Montana attorney and co-founder of A&M Law. Over two decades in the practice of law focusing on family law, criminal defense, and DUI defense, with federal and state court experience and a Certificate in Mediation.

Montana Bar #122346+ years as an attorney, with over two decades working in the lawReviewed by Ashley HurlbertLast reviewed: June 22, 2026

Key Takeaways

Montana sets a per se limit of 5 nanograms per milliliter (ng/mL) of delta-9-THC in whole blood. At or above that level you can be charged with DUI even without other proof of impairment.
You can also be charged with a marijuana DUI based on actual impairment, at any THC level, if cannabis affected your ability to drive safely.
Recreational and medical marijuana are legal in Montana, but legal use is not a defense to driving impaired.
THC is measured through a blood test, not a standard breath test, and Montana's implied-consent law applies to that draw.
Because THC can linger in the blood long after impairment fades, the science is genuinely contestable, which is often the heart of the defense.
A&M Law has successfully resolved a number of THC DUI cases and prepares each one as if it will go to trial.

Marijuana is legal in Montana, but driving while impaired by it is not. As cannabis use has become more common, so have THC DUI charges, and they have quickly grown into one of the areas of impaired-driving law we are asked about most. A&M Law has successfully resolved a number of these cases, and we have seen firsthand how different they are from a standard alcohol DUI.

The Legal Standard: Montana's 5 ng/mL THC Limit

Montana addresses marijuana-impaired driving in two distinct ways, and you can be charged under either one.

The per se THC limit. Montana sets a per se limit of 5 nanograms per milliliter (ng/mL) of delta-9-THC in whole blood. If a blood test places you at or above 5 ng/mL, the State can charge you with DUI per se. As with the 0.08% alcohol standard, per se means the prosecution does not have to separately prove that you were actually impaired. The number alone is the offense.
The impairment standard. Independent of any THC number, Montana makes it unlawful to drive while under the influence of any drug, including cannabis, to a degree that affects your ability to drive safely. Under this theory you can be charged at any THC level, including below 5 ng/mL, if the officer concludes that marijuana impaired your driving.

Montana recodified its impaired-driving statutes in 2021 (Title 61, Chapter 8, Part 10 of the Montana Code Annotated), and the marijuana-DUI provisions live within that framework alongside the alcohol rules. The penalties track the same structure: a first, second, and third offense are generally misdemeanors, while a fourth or subsequent offense becomes a felony.

Why THC Cases Are Different From Alcohol Cases

Alcohol leaves the body at a fairly predictable rate, which is what makes a breath or blood number a reasonable proxy for impairment at the time of driving. THC behaves very differently. Delta-9-THC can be detected in the blood of regular users hours or even days after use, long after any impairing effect has worn off. As a result, a number at or above 5 ng/mL does not always mean a person was impaired when they were behind the wheel.

That gap between what the test measures and what actually happened on the road is often the heart of a THC DUI defense. It is also why legal or medical cannabis use is not, by itself, a shield: legality governs whether you may possess and use marijuana, not whether you may drive after doing so.

How These Charges Are Proven

THC is not measured by a standard roadside breath test. Proving a marijuana DUI typically depends on a combination of:

A blood draw analyzed for delta-9-THC, governed by Montana's implied-consent law
Observations from the traffic stop and any field sobriety testing
An evaluation by a Drug Recognition Expert (DRE), where one is used
Officer testimony about driving behavior, odor, and physical signs

Each of these has weak points. Blood draws have to be timed, handled, stored, and tested correctly. DRE protocols are detailed and frequently applied imperfectly. And field sobriety tests were designed for alcohol, not cannabis, which raises real questions about what they actually show.

How A&M Law Defends THC DUI Cases

Misty Gaubatz is certified in NHTSA Standardized Field Sobriety Tests, the same certification the arresting officer holds, and she brings a prosecutor's understanding of how the State assembles a drug-DUI case. We prepare every matter as though it will go to trial, and we routinely examine:

Whether the officer had a lawful basis for the stop
What the THC result genuinely establishes about impairment at the time of driving
How the blood sample was drawn, stored, and tested, and whether the chain of custody holds up
Whether any drug-recognition evaluation followed proper protocol
Whether your constitutional rights were respected throughout

That preparation is what creates leverage, whether a case is resolved through a motion to suppress, a negotiated reduction, or a verdict.

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FAQ

Montana THC DUI, Frequently Asked Questions

What is the legal THC limit for driving in Montana?

Montana law sets a per se limit of 5 nanograms per milliliter (ng/mL) of delta-9-THC in whole blood. If a blood test shows you at or above 5 ng/mL, the State can charge you with DUI per se, meaning it does not have to separately prove you were impaired. You can also be charged below that level under the general impairment standard.

Can I get a DUI for marijuana even if I am under 5 ng/mL?

Yes. Separate from the per se limit, Montana makes it illegal to drive while actually impaired by any drug, including cannabis. If an officer believes marijuana affected your driving, you can be charged based on impairment regardless of your THC number.

Marijuana is legal in Montana. How can it still be a DUI?

Recreational and medical cannabis are legal to possess and use in Montana, but it is still illegal to drive while impaired by them, just as it is with alcohol or prescription medication. Having a medical card or using cannabis legally is not a defense to a charge of impaired driving.

Does THC in a blood test prove I was impaired when I was driving?

Not necessarily, and this is often the central issue in these cases. Unlike alcohol, THC does not clear the body on a predictable curve, and regular users can show measurable delta-9-THC hours or even days after use, well after any impairment has passed. The timing of the blood draw and what the number actually means can be challenged.

What happens if I refuse a blood test for THC?

Montana's implied-consent law applies to blood testing for drugs. Refusing can trigger an automatic license suspension separate from the criminal case, and officers can seek a warrant to compel a blood draw. Because deadlines to challenge a suspension are short, it is important to contact an attorney quickly.

How does A&M Law defend THC DUI cases?

We examine whether the traffic stop was lawful, how any drug-recognition evaluation was conducted, how the blood sample was drawn, stored, and tested, and what the THC result genuinely establishes about impairment at the time of driving. Misty Gaubatz is NHTSA/SFST certified and brings a prosecutor's understanding of how these cases are built.

Charged With a THC DUI? Call Before You Talk to Anyone.

The science is contestable and the deadlines are short. Call 406-830-3060 for a free, confidential consultation.

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