Last verified: March 2026
Montana's Cannabis DUI Standard
Montana uses a per se THC limit for impaired driving: 5 nanograms of active (delta-9) THC per milliliter of blood. This means that if a blood test shows 5 ng/mL or higher, you are legally impaired regardless of whether you appear impaired. Montana has had this per se standard since October 2013 — predating recreational legalization by seven years.
A person is under the influence of a dangerous drug if the person's blood contains 5 nanograms or more of delta-9-tetrahydrocannabinol per milliliter of blood.
Montana Code Annotated 61-8-1002
Key Points About the Per Se Standard
- Active THC only: Montana tests for delta-9 THC (the active compound), not THC-COOH (the inactive metabolite that can persist for weeks). This is more scientifically sound than metabolite-based testing.
- No impairment required: Unlike alcohol impairment (which requires observable signs for DUI charges below 0.08 BAC), exceeding 5 ng/mL THC is automatic legal impairment.
- Medical cards do NOT exempt you: Holding a medical cannabis card provides no protection against DUI charges. The per se limit applies equally to all drivers.
- Blood draw required: Unlike alcohol breathalyzers, THC testing requires a blood draw, which typically requires consent or a warrant.
Zero Tolerance for Under 21
Any detectable amount of THC in a driver under 21 constitutes a DUI in Montana. SB 508, signed in 2025, closed a gap that previously allowed under-21 drivers to have THC levels below the 5 ng/mL standard. There is no "safe amount" for drivers under 21.
DUI Penalties
| Offense | Jail Time | Fine | License Suspension |
|---|---|---|---|
| 1st offense | 24 hours – 6 months | $600 – $1,000 | 6 months |
| 2nd offense | 7 days – 1 year | $1,200 – $2,000 | 1 year |
| 3rd offense | 30 days – 1 year | $2,500 – $5,000 | 1 year |
| 4th+ offense | Felony — state prison up to 5 years, $10,000 fine | ||
A fourth or subsequent DUI is a felony in Montana. All DUI convictions also require completion of an alcohol/drug education or treatment program. Additional penalties may include ignition interlock device installation and community service.
How Long Does THC Stay in Blood?
Active delta-9 THC concentrations in blood vary significantly based on consumption method and individual factors:
- Smoking/vaping: THC peaks in blood within minutes and typically drops below 5 ng/mL within 2–4 hours for occasional users, but may remain elevated longer in heavy daily users.
- Edibles: THC peaks in blood 1–3 hours after consumption and declines more slowly. Effects and blood levels from edibles are less predictable.
- Heavy daily users: Chronic consumers may have baseline THC levels above 5 ng/mL even when not acutely impaired. This is a known limitation of per se standards.
Wait at least 4–6 hours after smoking or vaping and 6–8 hours after edibles before driving. Heavy daily users may need longer. When in doubt, use a rideshare service or plan ahead with a sober driver. Montana's vast distances make planning essential.
Transporting Cannabis in Your Vehicle
- Cannabis must be in sealed original packaging
- Store in the trunk or glove box — not on seats or in the open
- Open or partially consumed cannabis in the passenger area is treated like an open container
- No consumption by any vehicle occupant while the vehicle is in motion
Implied Consent
Montana has an implied consent law for driving: by operating a motor vehicle, you consent to a blood or breath test if an officer has probable cause to believe you are impaired. Refusal to submit to testing can result in license suspension and can be used as evidence against you.
Official Sources
For in-depth cannabis education, dosing guides, safety information, and research summaries, visit our partner site TryCannabis.org