In Montana, "under the influence" has a statutory definition: as a result of taking alcohol, drugs, or a combination into the body, "a person's ability to safely operate a vehicle has been diminished" (MCA § 61-1-101). That one sentence is the standard for the impairment form of DUI. This page explains what it means and what evidence the State uses to prove it. It is part of our Montana DUI Guide.
Where the Definition Sits
The impairment offense is MCA § 61-8-1002(1)(a): driving, or being in actual physical control of, a vehicle on the ways of this state open to the public "while under the influence of alcohol, any drug, or a combination of alcohol and any drug." The DUI part of the code says "under the influence" has the meaning provided in § 61-1-101 (MCA § 61-8-1001(14)), which is the sentence quoted above.
This is separate from the offenses defined by a number, such as 0.08. Those do not use the phrase at all. See per se vs. impairment DUI.
Reading the Definition Word by Word
- "As a result of taking into the body." The diminished ability has to be caused by alcohol or a drug. Fatigue, illness, injury, or nerves are not the offense, though they can look similar at the roadside.
- "Alcohol, drugs, or any combination." A small amount of alcohol with a medication can meet the definition together even if neither would alone.
- "Ability to safely operate a vehicle." The focus is on capacity, not on whether anything bad happened.
- "Has been diminished." Reduced, not eliminated. The statute sets no degree.
That last point is the one people underestimate. NHTSA's officer training manual tells officers that every state makes it unlawful to drive under the influence and that "a particular person may be under the influence at much lower BACs" than 0.08.
What Counts as a Drug
For DUI purposes, a drug is "any substance that when taken into the human body can impair a person's ability to operate a vehicle safely" (MCA § 61-8-1001(7)). That reaches illegal drugs, marijuana, prescriptions, and over-the-counter medicine alike.
Because that definition is so broad, the Legislature added a limit. A positive test "does not, in itself, prove that the person was under the influence of a drug," and a person may not be convicted of an impairment DUI based on the presence of a drug "unless some other competent evidence exists that tends to establish that the person was under the influence" while driving (MCA § 61-8-1018(1)(a)). See over-the-counter medicine and drug recognition evaluations.
The Evidence Used to Show Diminished Ability
| Evidence | What it is | Source |
|---|---|---|
| Driving | What the officer reports seeing before the stop, and what the dash camera recorded. | General evidence; MCA § 61-8-1002(3) |
| Personal contact | Odor, speech, eyes, coordination, and how you answered questions. | General evidence; MCA § 61-8-1018(3) |
| Field sobriety tests | Admissible if the report was made by a person trained by the Department of Justice or with training it recognizes. | MCA § 61-8-1018(1)(c) |
| Breath, blood, or oral fluid result | Any measured amount or detected presence of alcohol or drugs is admissible. | MCA § 61-8-1018(1)(a) |
| Alcohol inferences | 0.04 or less: may infer not under the influence. 0.08 or more: may infer under the influence, rebuttably. | MCA § 61-8-1002(2) |
| A refusal | Proof of refusal is admissible, and the trier of fact may infer the person was under the influence. The inference is rebuttable. | MCA § 61-8-1018(2) |
Twice the code says its specific rules do not limit "any other competent evidence" on whether a person was under the influence (MCA §§ 61-8-1002(3), 61-8-1018(3)). That cuts both ways. The defense may offer competent evidence too: a passenger's account, a receipt showing one drink, medical records, or video.
How the Inferences Work
An inference is "a deduction which the trier of fact may make from the evidence" (MCA § 26-1-501). The DUI statutes create three that bear on this definition:
- An alcohol concentration of 0.04 or less: it may be inferred the person was not under the influence of alcohol.
- An alcohol concentration of 0.08 or more: it may be inferred the person was under the influence of alcohol. "The inference is rebuttable."
- A refusal to test: the trier of fact may infer the person was under the influence. That inference is rebuttable as well.
Between 0.04 and 0.08 there is no inference in either direction (MCA § 61-8-1002(2)(b)). The alcohol inferences apply to a sample taken within a reasonable time after the driving. None of them shifts the burden of proof to the driver. See DUI under the limit.
What the Definition Does Not Require
- Intent. The statute imposes absolute liability (MCA § 61-8-1002(5)). The State need not prove you knew you were affected.
- A crash or a near miss. Neither is an element.
- A number. A test result is evidence, not a requirement, for this form of the charge.
- Moving. Being in actual physical control of a vehicle is enough.
Where Impairment Cases Are Contested
Because the standard is about ability, the defense looks for evidence of ability. Did you pull over promptly and park normally? Did you find your license without fumbling, answer clearly, and walk steadily on video? Were the field sobriety tests given as designed, by an officer with the training the statute requires? Is there another cause for what the officer saw, such as a medical condition, an injury, or exhaustion?
An officer's opinion is evidence, but it is still opinion, and it can be checked against the video. Our page on challenging field sobriety tests goes further.
This page explains Montana law in general as of the 2025 Montana Code Annotated. Every case turns on its own facts. It is not legal advice about your specific case.