You can be charged with DUI in Montana for driving on a prescribed opioid, but only if the State can show the medication diminished your ability to drive safely. The blood limits Montana adopted in 2025 for certain opioids apply to people without a valid prescription. This page separates the two kinds of charge and explains how each works for pain medication. It is part of our Montana DUI Guide and builds on our overview of prescription and drug DUI.
Two Charges, Two Different Questions
Impairment. 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 any drug. The question is whether your ability to safely operate a vehicle was diminished (MCA § 61-1-101). Whether the drug was legal for you to take is not part of that question.
Per se.MCA § 61-8-1002(1)(f) makes it an offense to drive with a listed substance in your blood at or above a set level, "without a valid prescription." The question is the number and the prescription. Impairment does not have to be shown.
Which Opioids Have a Blood Limit
| Opioid | Per se blood level | Notes |
|---|---|---|
| Fentanyl | 0.5 ng/mL | Listed at § 61-8-1002(1)(f)(ix). Per se offense only without a valid prescription. |
| Heroin | 1 ng/mL | Listed at (1)(f)(iv). NHTSA notes heroin has no currently accepted medical use in the United States. |
| 6-monoacetyl morphine | 1 ng/mL | Listed as a heroin metabolite at (1)(f)(v)(B). |
| Morphine | 20 ng/mL | Listed as a heroin metabolite at (1)(f)(v)(A). Also a prescription drug and, per NHTSA, a metabolite of codeine. |
| Oxycodone, hydrocodone, hydromorphone, methadone, tramadol, buprenorphine | None | Not on the list. Impairment charge only. |
| Codeine | None for codeine itself | Not on the list, but NHTSA reports codeine is metabolized to morphine. |
The complete list, including the non-opioid entries, is on our page about Montana's drug limits.
What a Valid Prescription Does
For fentanyl and morphine, the prescription wording is the whole difference between a per se offense and no per se offense. A patient with a valid prescription for the drug in their blood is not covered by subsection (1)(f).
The statute leaves questions open, and it is better to know that than to assume:
- The DUI definitions in MCA § 61-8-1001 do not define "valid prescription."
- The text does not say how it treats a dose above what was prescribed, medication left over from an old prescription, or a pill taken from a family member's bottle.
- The text does not say who has to show that a prescription exists. Keep your records either way.
In practice, pharmacy records and the prescriber's chart are the first documents to gather.
The Morphine and Codeine Problem
Montana lists morphine under the heading "heroin metabolite," at 20 ng/mL. But morphine reaches the blood from more than one source. NHTSA's Drugs and Human Performance Fact Sheets: 2024state that "Morphine itself is also a metabolite of codeine, ethylmorphine, heroin, and pholcodine." The same fact sheets describe 6-acetylmorphine as a metabolite of heroin with a half-life measured in minutes, and Montana gives it a separate, much lower limit.
A laboratory report showing morphine and nothing else therefore does not, by itself, show heroin use. What else the laboratory found, and what the person was prescribed, are the facts that give the number meaning.
How Impairment Is Proved on Prescribed Opioids
For a patient with a prescription, or for any opioid that is not on the list, the case is an impairment case. Two rules protect against conviction on a test result alone. 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 tends to establish impairment (MCA § 61-8-1018(1)(a)).
That other evidence is usually the driving, the officer's observations, the field sobriety tests, statements, and sometimes an evaluation by an officer with drug recognition training. Chronic pain, an injury, or fatigue can affect balance and coordination for reasons unrelated to medication, and that belongs in the analysis.
The science on blood levels is cautious. NHTSA states that "Tolerance makes interpretation of blood or plasma morphine concentrations extremely difficult." Its summary of laboratory studies also notes that the overall effect of morphine on cancer or chronic pain patients appeared less pronounced. That does not mean a long-term patient cannot be impaired. It means the number alone does not settle the question.
Combinations Raise the Risk
Opioids are often taken alongside other sedating medication. In 2016 the FDA required boxed warnings on prescription opioid pain and cough medicines and on benzodiazepines about combined use, listing risks that include extreme sleepiness, respiratory depression, coma, and death. Alcohol adds to the picture. Montana's impairment offense expressly covers "a combination of alcohol and any drug," so a person can be charged when neither substance alone would have led to a charge. See alcohol and drug combinations.
If You Are a Pain Patient Facing a Charge
- Do not stop or change your medication without talking to your prescriber.
- Collect your prescription history, including dose changes and how long you have been on the current dose.
- Write down when you took each dose on the day of the arrest, and anything else you took.
- Note any medical condition that affects your balance, speech, or eyes.
- Ask for the full laboratory report, not just the summary. You are entitled to full information about a test given at an officer's direction (MCA § 61-8-1019(3)).
How the blood evidence can be examined is covered in our guide to challenging a blood test.
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.