Most people think of DUI as an alcohol offense. In Montana it is broader than that. A charge can be based on a sleep aid, a pain medication, an anti-anxiety drug, or an illegal substance, and the person charged is often someone who took a prescription exactly as directed. This page explains how Montana treats drug-based DUI. It is part of our Montana DUI Guide; marijuana has its own page.
Two Kinds of Drug DUI Charge
Impairment.Under MCA § 61-8-1002(1)(a), a person commits DUI by driving or being in actual physical control of a vehicle while under the influence of alcohol, any drug, or a combination. "Drug" means any substance that when taken into the body can impair a person's ability to operate a vehicle safely (MCA § 61-8-1001(7)). Nothing in that definition turns on whether the substance was legal for you to take.
Per se. Under MCA § 61-8-1002(1)(f), added by the 2025 Legislature and in effect since October 1, 2025, a person commits DUI by driving with a listed substance in their blood at or above a set level, without a valid prescription. Here the State does not have to prove diminished ability, only the number.
The 2025 Per Se Drug Limits
| Substance | Blood level (ng/mL) |
|---|---|
| Amphetamine | 20 |
| Cocaine | 20 |
| Cocaine metabolite | 20 |
| Heroin | 1 |
| Morphine (heroin metabolite) | 20 |
| 6-monoacetyl morphine (heroin metabolite) | 1 |
| Lysergic acid diethylamide (LSD) | 0.1 |
| Methamphetamine | 20 |
| Phencyclidine (PCP) | 5 |
| Fentanyl | 0.5 |
Source: MCA § 61-8-1002(1)(f). Delta-9-THC is handled separately, at 5 ng/mL for adults under § 61-8-1002(1)(d) and at any amount for a driver under 21.
What a Valid Prescription Does, and Does Not, Do
- It takes you out of the per se drug limits in § 61-8-1002(1)(f), which apply only "without a valid prescription."
- It does not prevent an impairment charge under § 61-8-1002(1)(a). If the State can show the medication diminished your ability to drive safely, the prescription is not a defense to that.
- It can still matter to the case. The dose, how long you have taken it, and what the label and your prescriber told you all bear on whether you were actually impaired.
A Positive Test Is Not Proof of Impairment
This is the most important rule in a prescription case. MCA § 61-8-1018(1)(a) says a positive test result "does not, in itself, prove that the person was under the influence of a drug or drugs." A person may not be convicted of an impairment DUI based on the presence of a drug unless some other competent evidence tends to establish that the person was under the influence while driving or in actual physical control.
In practice that "other evidence" is the officer's account of the driving, the roadside observations, the field sobriety tests, and any statements. Those tests were developed and validated for alcohol, which is one of the things a defense looks at closely in a drug case.
How Drug DUI Cases Are Tested
A breath test measures alcohol only. A drug case nearly always depends on a blood sample. Under Montana's implied consent law, a driver arrested for DUI is treated as having consented to a test of blood, breath, or oral fluid (MCA § 61-8-1016), and refusing has its own license consequences, which our test refusal page covers.
For a blood result to come in, the sample must have been drawn by a person competent to do so and analyzed in a laboratory operated or certified by the Department of Justice, or one exempt from certification (MCA § 61-8-1018(1)(b)). Depending on the facts, a defense examines:
- whether the stop and the arrest were lawful;
- who drew the blood, how it was labeled and stored, and the chain of custody;
- what the laboratory actually measured, and whether it reported an active drug or an inactive metabolite;
- whether there is competent evidence of impairment apart from the test;
- for a per se charge, whether there was a valid prescription.
Penalties Are the Same as Any DUI
A drug-based DUI is sentenced under the same statutes as an alcohol DUI. A first per se drug offense carries up to 6 months in jail with no jail minimum and a fine of $600 to $1,000, while a first impairment offense carries a 24-hour minimum (MCA § 61-8-1007). The license suspension, the chemical dependency assessment, and the way the conviction counts as a prior are all the same.
This page explains Montana law in general as of the 2025 Montana Code Annotated. It is not legal or medical advice. Do not stop or change a prescribed medication without talking to your prescriber.