A Montana medical marijuana card does not exempt you from a DUI charge. The card makes it lawful to possess and use marijuana within set amounts. It does not make it lawful to drive under the influence of marijuana or to drive with 5 ng/mL or more of THC in your blood. This page walks through the statutes that say so and explains what the card still does for you. It is part of our Montana DUI Guide, and it goes deeper on one question raised on our THC DUI page.
What the Marijuana Law Says About Driving
Montana's marijuana chapter protects cardholders in broad terms. Under MCA § 16-12-515(2), a person who holds a registry identification card may not be arrested, prosecuted, or penalized solely because the person possesses or transports marijuana in the allowed amounts, or because the cardholder acquires or uses marijuana.
That protection opens with the words "Except as provided in 16-12-108." Section 16-12-108 is titled "Limitations of act," and its first three items concern vehicles. The chapter does not permit:
- operating or being in actual physical control of a motor vehicle, train, aircraft, motorboat, or other motorized form of transport while under the influence of marijuana or marijuana products;
- consuming marijuana while operating or being in physical control of a motor vehicle while it is being operated;
- smoking or consuming marijuana while riding in the passenger seat within an enclosed compartment of a motor vehicle while it is being operated.
So the cardholder protection stops where impaired driving starts. The same limits apply to recreational users age 21 and over.
What the DUI Statute Says
The DUI statute reaches marijuana in two ways. The impairment offense covers "any drug" (MCA § 61-8-1002(1)(a)), and a drug is any substance that can impair a person's ability to operate a vehicle safely (MCA § 61-8-1001(7)). The per se offense applies when a person's "tetrahydrocannabinol level, excluding inactive metabolites, as shown by analysis of the person's blood or other bodily substance, is 5 ng/ml or more" (MCA § 61-8-1002(1)(d)).
Neither provision mentions a registry card. The statute also makes DUI an absolute liability offense (MCA § 61-8-1002(5)), so the State does not have to prove that you knew you were over a limit.
Why the Card Is Not a "Valid Prescription"
People sometimes read about Montana's 2025 drug limitsand see that they apply only to a person "without a valid prescription." That wording sits in subsection (1)(f), which lists drugs such as fentanyl, methamphetamine, and amphetamine. THC is not on that list. It has its own subsection, and that subsection has no prescription wording.
There is also a practical difference. A cardholder does not receive a prescription. The marijuana chapter works through a physician's "written certification" and a registry card issued by the Department (MCA §§ 16-12-508 and 16-12-509).
What a Card Does and Does Not Protect
| Situation | Effect of the card | MCA |
|---|---|---|
| Possessing and using marijuana within the legal amounts | Protected | § 16-12-515(2) |
| Being searched only because you hold a card | Card alone is not probable cause | § 16-12-515(6) |
| Driving under the influence of marijuana | Not protected | § 16-12-108(1)(a) |
| Driving at 5 ng/mL of THC or more | Not protected; no card or prescription wording | § 61-8-1002(1)(d) |
| Using marijuana while driving or riding in a moving vehicle | Not protected | § 16-12-108(1)(b), (c) |
| Open marijuana in the passenger area on a highway | No cardholder exception in the text | § 61-8-1027 |
A cardholder must keep the card in their immediate possession and show it, with photo identification, on demand of a law enforcement officer (MCA § 16-12-513(1)). The rules for carrying marijuana in a vehicle are on our page about marijuana in the car.
Cardholders Under 21
Since October 1, 2025, a driver under 21 commits an offense with "any amount of tetrahydrocannabinol, excluding inactive metabolites," in their body (MCA § 61-8-1002(1)(e)(ii)). The marijuana chapter allows some people under 21 to be registered cardholders, but the under-21 driving provision contains no exception for them. Read literally, a young cardholder who uses lawfully can still be over the driving limit at any measurable level of active THC. Our under-21 DUI page covers the penalties, which are lower than for an adult DUI.
Why Daily Medical Use Makes These Cases Different
A patient who uses marijuana every day is in a different position from an occasional user when blood is drawn. NHTSA's Drugs and Human Performance Fact Sheets (2024) report that residual THC may be detected in the blood of some chronic, frequent users for an extended period after they stop, and state that it is difficult to establish a relationship between a person's THC blood concentration and performance impairing effects.
That does not create an exemption from the per se limit. It does shape the defense. The questions are when the blood was drawn, what the laboratory reported and whether it separated active THC from inactive metabolites, and, for an impairment charge, whether there is competent evidence of impairment apart from the test (MCA § 61-8-1018(1)(a)). See how long THC stays in your blood.
If You Are a Cardholder and Were Arrested
The card does not need to be hidden, and it is not an admission of impairment. What you say about when and how much you last used is a different matter, and it is often the State's main evidence of timing. A person arrested for DUI is treated as having consented to a blood test, and refusing has license consequences of its own, covered on our test refusal page.
This page explains Montana law in general as of the 2025 Montana Code Annotated. It is not legal or medical advice about your specific case.