Yes, an over-the-counter medicine can lead to a DUI in Montana. The statute covers any substance that can impair your ability to drive safely, and it does not distinguish between a prescription, a product from a pharmacy shelf, and an illegal drug. What the State cannot do is convict you because a medicine was in your system. It has to prove impairment. This page explains how that works for allergy, cold, and sleep products. It is part of our Montana DUI Guide.
Why Montana's DUI Law Reaches Shelf Medicine
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)). For this part of the code, "drug" means "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 definition turns on effect, not legal status. A sedating antihistamine fits it. So does a nighttime sleep aid, and so can a cough suppressant taken in a large enough amount.
The Medicines That Come Up Most
NHTSA's Drugs and Human Performance Fact Sheets: 2024 include two ingredients sold without a prescription.
| Ingredient | Found in (per NHTSA) | How NHTSA describes it |
|---|---|---|
| Diphenhydramine | Benadryl, Unisom SleepGels, Sominex; an ingredient in many nighttime cold and pain products | Antihistamine, sleep aid, CNS depressant. Effects usually last 4 to 6 hours. |
| Dimenhydrinate | Dramamine | A combination containing diphenhydramine, used for motion sickness. |
| Dextromethorphan | Robitussin, Delsym, Vicks Formula 44, many "DM" products | Cough suppressant. CNS depressant in high doses. Duration about 3 to 6 hours at clinical doses. |
The FDA's consumer guidance on medicines and driving adds a broader list of products that can make driving dangerous. It includes cold remedies and allergy medicines that contain an antihistamine, nighttime sleep aids, cough medicines, motion sickness medicines, and stimulants such as pseudoephedrine.
What the Research Says About Diphenhydramine
Diphenhydramine is the ingredient with the strongest published record. NHTSA's fact sheets report:
- Laboratory studies have shown therapeutic doses decrease alertness, impair concentration and tracking, and affect attention and memory within the first 2 to 4 hours after a dose.
- In a study on the National Advanced Driving Simulator, a single 50 mg dose was compared with an alcohol concentration of 0.10. Overall driving performance was poorest after diphenhydramine, and the authors concluded it "may have an even greater impact than does ethanol" on operating a motor vehicle.
- A review of on-the-road studies concluded that first-generation antihistamines, including diphenhydramine, significantly impaired driving performance.
- One study found no significant driving effect during a short, low-speed driving test, although the same doses affected laboratory performance.
The FDA puts it more simply. Its consumer guidance says antihistamines can slow reaction time, make it hard to focus or think clearly, and cause mild confusion even in a person who does not feel drowsy.
Products and labels differ. The Drug Facts label on the package you actually took is the place to check.
What the State Would Have to Prove
Because no over-the-counter ingredient is on the per se drug list, there is no shortcut for the State. It must prove that your ability to safely operate a vehicle was diminished (MCA § 61-1-101). The evidence statute adds two protections: 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 other competent evidence tends to establish impairment (MCA § 61-8-1018(1)(a)).
In practice the State's evidence is the driving, the officer's observations, the field sobriety tests, and what the driver said. Someone with a bad cold or allergies may have red, watery eyes, a thick voice, and fatigue for reasons that have nothing to do with impairment, and that context matters when the officer's report is read.
Whether a laboratory tests a blood sample for a particular over-the-counter ingredient depends on the testing requested and performed. It should be confirmed from the report, not assumed.
The Real Risk: Mixing With Alcohol
The situation to watch for is a drink or two on top of a nighttime cold medicine. NHTSA's fact sheets describe two controlled studies in which diphenhydramine and alcohol each impaired performance and the combination impaired it further "in an additive manner." In one of them the average breath alcohol concentration was 0.043.
Montana's impairment offense expressly includes "a combination of alcohol and any drug." A breath result under 0.08 does not end the inquiry if the officer believes something else was involved. See our pages on alcohol and drug combinations and DUI under the limit.
"It Was Legal" Is Not the Defense
DUI is an absolute liability offense in Montana (MCA § 61-8-1002(5)). The State does not have to prove you knew a medicine would affect you. The defenses that matter are factual: that you were not impaired, that the signs the officer saw have another explanation, or that the stop or the testing was unlawful.
A sleep aid raises one more issue. Prescription sleep medications carry FDA warnings about next-morning impairment, and NHTSA lists diphenhydramine as an ingredient in over-the-counter sleep aids. Our page on Ambien and sleep-driving covers sleep medication in more detail.
If You Were Arrested After Taking a Medicine
- Keep the package, or photograph the Drug Facts label, including the dose and warnings.
- Write down what you took, how much, and when, along with anything you drank.
- Note the illness or allergy you were treating and its symptoms that day.
- Do not guess aloud about whether the medicine "hit you hard." Statements become evidence.
The general rules for medication cases are in our guide to prescription and drug DUI.
This page explains Montana law in general as of the 2025 Montana Code Annotated and summarizes published federal sources. It is not legal or medical advice. Read the label and ask a pharmacist or doctor about any medicine and driving.