Alcohol Plus a Drug: Combination DUI in Montana

You can be convicted of DUI in Montana when you are under 0.08 for alcohol and under the limit for any drug, if the State proves that the combination diminished your ability to drive safely. The statute says so in its first subsection. This page explains how a combination case is charged and proved, and where it is weak. It is part of our Montana DUI Guide.

The Statute Names the Combination

MCA § 61-8-1002(1)(a) makes it an offense to drive or be in actual physical control of a vehicle "while under the influence of alcohol, any drug, or a combination of alcohol and any drug." Under the influence means that "as a result of taking into the body alcohol, drugs, or any combination of alcohol and drugs, a person's ability to safely operate a vehicle has been diminished" (MCA § 61-1-101).

Neither provision sets a number for a combination. There is no formula that adds an alcohol result to a THC result. The question is the effect on the driver.

How It Differs From the Per Se Offenses

The penalty difference is easy to miss. A first conviction under the impairment subsection carries imprisonment of "not less than 24 consecutive hours" (MCA § 61-8-1007(1)(a)(i)). A first conviction under a per se subsection carries up to 6 months with no minimum (MCA § 61-8-1007(2)(a)(i)). A person who is under every limit and convicted on the combination theory therefore faces a mandatory minimum that a person over 0.08 on a per se count does not. Our page on per se and impairment DUI compares the two in full.

What a Low Alcohol Result Does and Does Not Do

The statute attaches inferences to an alcohol test in a case alleging the person was under the influence of alcohol (MCA § 61-8-1002(2)):

• at 0.04 or less, it may be inferred that the person "was not under the influence of alcohol";
• above 0.04 and below 0.08, there is no inference either way;
• at 0.08 or more, it may be inferred that the person was under the influence of alcohol, and the inference is rebuttable.

Each inference is about alcohol. Subsection (3) adds that these provisions "do not limit the introduction of any other competent evidence bearing on the issue of whether the person was under the influence of alcohol, drugs, or a combination of alcohol and drugs." So a low breath result is helpful evidence, but in a combination case the State will argue that it does not answer the question being asked. How much weight the low result deserves is for the judge or jury.

Why Combinations Are Treated Seriously

The legal rule tracks what the research describes. NHTSA's Drugs and Human Performance Fact Sheets: 2024 report:

• Alcohol and THC. "Low doses of THC and ethanol that may not affect psychomotor function when given alone may still impair performance when given in combination."
• Alcohol and diphenhydramine (the antihistamine in Benadryl). In controlled studies, each impaired performance alone and the combination impaired it further in an additive manner.
• Alcohol and zolpidem (Ambien). Additive effects of alcohol were detected with a 10 mg dose in a controlled study.
• Alcohol and diazepam (Valium). Alcohol added to the impairing effects of diazepam.

Separately, in 2016 the FDA required boxed warnings about combining opioids with benzodiazepines, listing risks that include extreme sleepiness and respiratory depression.

These are population findings. They do not prove that a particular person was impaired on a particular night, and they should not be read as describing a safe amount of anything.

What the State Has to Show

A combination case is an impairment case, with the limits that come with one. A positive test "does not, in itself, prove that the person was under the influence of a drug or drugs," and 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 (MCA § 61-8-1018(1)(a)).

The State typically offers:

• the driving that led to the stop, and any crash;
• the officer's observations and video;
• field sobriety test results;
• the breath result and a blood toxicology report;
• statements about what was consumed and when;
• sometimes a drug recognition evaluation or testimony from a toxicologist.

Where Combination Cases Are Contested

Because there is no number, the argument is about inference, and several points recur.

Timing. A drug in blood does not show when it was taken. THC in particular can remain measurable in frequent users long after use, as our page on how long THC stays in your blood explains. A medication taken the night before is a different fact from one taken an hour before driving.

Active drug or metabolite. A report may list inactive breakdown products. Their presence shows past exposure.

Other explanations. Fatigue, illness, injury, and nervousness affect roadside performance. The field tests were validated for alcohol.

The lawfulness of the second test. Whether the officer had grounds to continue the investigation after a low breath result, and how the blood was obtained, can be challenged. See DUI under the limit and challenging a blood test.

Charged Under More Than One Theory

When a result is near a limit, the State may charge both a per se count and an impairment count. Subsection (6) permits that and caps the outcome: a person "may be convicted of only one offense under this section." Which count a case resolves on affects the minimum sentence, so it is a real subject of negotiation and trial strategy, not a technicality.

This page explains Montana law in general as of the 2025 Montana Code Annotated and summarizes published federal research. It is not legal or medical advice about your specific case.