Xanax, Valium, Klonopin, and other benzodiazepines can lead to a DUI in Montana, but only on an impairment theory. Montana has no blood limit for them. The State has to prove that the medication, alone or with something else, diminished your ability to drive safely. This page explains how that proof works. It is part of our Montana DUI Guide and goes deeper than our overview of prescription and drug DUI.
Benzodiazepines Are Not on the Per Se List
In 2025 Montana adopted blood limits for a short list of drugs, in effect since October 1, 2025 (MCA § 61-8-1002(1)(f)). The list is amphetamine, cocaine and its metabolite, heroin and two metabolites, LSD, methamphetamine, PCP, and fentanyl. No benzodiazepine appears on it.
Two consequences follow. A patient does not need the statute's "valid prescription" wording, because there is no per se benzodiazepine offense to be exempted from. And a person without a prescription is not guilty of DUI merely because a benzodiazepine shows up in their blood.
The Charge Is Impairment
The impairment offense applies to a person who drives or is in actual physical control of a vehicle "while under the influence of alcohol, any drug, or a combination of alcohol and any drug" (MCA § 61-8-1002(1)(a)). A drug is any substance that can impair a person's ability to operate a vehicle safely (MCA § 61-8-1001(7)). "Under the influence" means that ability has been diminished (MCA § 61-1-101).
A first conviction for the impairment offense carries a minimum of 24 consecutive hours in jail and a fine of $600 to $1,000 (MCA § 61-8-1007(1)(a)). It is sentenced and counted as a prior exactly like an alcohol DUI.
What NHTSA Says These Drugs Do
NHTSA's Drugs and Human Performance Fact Sheets: 2024 group benzodiazepines as central nervous system depressants used for anxiety, muscle spasm, seizures, and sedation. They classify the common ones by how long they act:
| Generic name | Common brand | Half-life class (NHTSA) |
|---|---|---|
| Alprazolam | Xanax | Short to intermediate |
| Lorazepam | Ativan | Intermediate |
| Clonazepam | Klonopin | Long |
| Diazepam | Valium | Long |
The fact sheets state that benzodiazepines "can produce a state of intoxication like that of ethanol, including slurred speech, disorientation, and severe sedation." For alprazolam, they report main effects lasting up to 6 to 8 hours after a dose.
That overlap with alcohol explains how many of these cases start. An officer sees weaving or slow speech, expects alcohol, gets a low breath result, and then asks for blood.
Why the Blood Number Proves Little
Montana's evidence statute says a positive test "does not, in itself, prove that the person was under the influence of a drug or drugs at the time the person was in control of a vehicle," and bars an impairment conviction based on the presence of a drug unless other competent evidence tends to establish impairment (MCA § 61-8-1018(1)(a)).
The science supports that caution. NHTSA states that "blood concentrations do not always provide a clear indication of behavioral effects," that concentrations may be several-fold higher after chronic use than after a single use, and that interpreting a level without knowing the person's history of use "is not advisable." In a review of on-the-road studies the fact sheets describe, measured blood concentrations of benzodiazepines were not reliably predictive of changes in driving performance.
On tolerance, NHTSA reports that regular long-term use produces tolerance to many of the sedative effects, and it also describes a study in which long-term users still showed reduced vigilance and more weaving in road tests. Tolerance is a fact to weigh, not a shield.
What the State Uses Instead
Because the number does not carry the case, the State relies on:
- the driving that led to the stop;
- the officer's observations of speech, balance, and alertness, and any body camera video;
- performance on field sobriety tests, which were developed and validated for alcohol;
- statements about what was taken and when;
- sometimes an evaluation by an officer with drug recognition training.
Each can be examined. A defense will often ask whether nervousness, fatigue, or a medical condition, and not the medication, explains what the officer saw.
Alcohol and Opioid Combinations
The impairment offense covers a combination of alcohol and a drug, so a person can be charged with an alcohol concentration below 0.08. NHTSA's fact sheets describe controlled studies in which alcohol added to the performance effects of diazepam and alprazolam.
In 2016 the FDA required boxed warnings on benzodiazepines and on prescription opioid pain and cough medicines about using them together, listing risks that include extreme sleepiness, respiratory depression, coma, and death. If your blood test shows more than one substance, read our pages on alcohol and drug combinations and opioids and pain medication.
If You Take a Benzodiazepine and Were Arrested
Do not stop the medication on your own. Stopping a benzodiazepine abruptly is a medical decision for your prescriber. Gather your prescription history, note the time of your last dose and anything else you took that day, and avoid guessing about amounts when asked. The blood sample and the laboratory report can be reviewed, as our guide to challenging a blood test explains.
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.