There is no fixed number of hours that THC stays in blood. In an occasional user it can drop below measurable levels within hours of smoking. In a daily user it can remain measurable for days. That uncertainty is why a blood number in a marijuana DUI means something different from a breath number in an alcohol case. This page explains what Montana's limit measures and what the federal research says about timing. It is part of our Montana DUI Guide.
The science below comes from the National Highway Traffic Safety Administration's Drugs and Human Performance Fact Sheets: 2024 (DOT HS 813 650). Nothing here is a guide to when it is safe or legal to drive after using marijuana.
What Montana's 5 ng/mL Limit Measures
A person commits a per se DUI when their "tetrahydrocannabinol level, excluding inactive metabolites, as shown by analysis of the person's blood or other bodily substance, is 5 ng/ml or more" (MCA § 61-8-1002(1)(d)). For a driver under 21, the offense is "any amount" of THC, again excluding inactive metabolites (MCA § 61-8-1002(1)(e)(ii)).
Three compounds usually appear on a cannabis toxicology report:
| Compound | What it is (per NHTSA) | Does it count toward the limit? |
|---|---|---|
| Delta-9-THC | The main psychoactive compound in marijuana. | Yes. This is the active compound a blood test reports. |
| 11-OH-THC | A metabolite the liver makes from THC. NHTSA describes it as equipotent to THC. | The statute excludes only "inactive" metabolites. It does not name this one. |
| THC-COOH | A later metabolite. NHTSA describes it as not psychoactive and as detectable far longer than THC. | No. It is an inactive metabolite, which the statute excludes. |
The statute does not list the metabolites by name. How a particular laboratory reports each compound, and which figure the State relies on, is something to read off the actual report. Our guide to crime lab and toxicology reports explains how.
After Smoking: A Fast Peak and a Fast Fall
NHTSA reports that THC is absorbed rapidly during smoking or vaporization, with peak blood concentrations occurring within 5 to 10 minutes. Concentrations then decline quickly. In one study the fact sheets describe, average THC concentrations 15 and 30 minutes after smoking were about 60 percent and 20 percent of the peak. In another, blood THC fell about 74 percent within 30 minutes of the peak and 90 percent within 1.4 hours.
In occasional smokers in one study, blood THC was detected for a median of only 4 hours after a single cigarette, with a range of 1 to 6 hours.
Frequent Users: THC That Lingers
THC is stored in body fat and released slowly. NHTSA's fact sheets report that because of this, residual THC may be detected in some chronic, frequent smokers' blood for up to 30 days after they stop, at very low levels. In one study the fact sheets cite, 9 of 11 frequent users were still at 1 ng/mL or more on each of 7 days of abstinence, and one had a blood THC concentration of 5 ng/mL or more for as long as 5 days.
For a daily user, including a medical marijuana patient, this means a blood result can be above zero, and in some people near or above the per se figure, at a time when the person has not used recently. The per se statute has no exception for that situation. It is a fact for the defense to develop, not an automatic answer to the charge.
Why the Timing of the Blood Draw Matters
NHTSA notes that in driving cases blood is typically collected about 1 to 4 hours after the incident, with an unknown time of last use, and that this "complicates interpretation" because THC concentrations decrease rapidly after the last inhalation.
The delay works in two directions:
- The number at the draw is not the number at the wheel. For someone who used shortly before driving, the level while driving was probably higher than the report shows. The State cannot reconstruct it, because NHTSA says the science does not permit back-calculation of a THC result.
- A low or moderate number does not date the use. The same result could reflect recent use by an occasional user or residual THC in a frequent one.
The per se offense is written in terms of the level "as shown by analysis," and blood is drawn after an arrest, not during the driving. How long the draw took, and why, is part of every THC case. See our pages on blood test procedure and blood draw warrants.
A Blood Level Is Not a Measure of Impairment
NHTSA states: "It is difficult to establish a relationship between a person's delta-9-THC blood or plasma concentration and performance impairing effects." The fact sheets add that a person can be affected by marijuana with THC below the laboratory's reporting limit, and they describe a study in which the 5 ng/mL per se cutoff "showed limited relevance in discriminating impaired from non-impaired drivers."
On duration of effects, NHTSA reports that the effects of smoked cannabis peak in 10 to 30 minutes, that most behavioral and physiological effects return to baseline within 3 to 4 hours, and that some investigators found residual effects on specific tasks for up to 24 hours. It also notes that psychomotor impairment can persist after the perceived high has faded. Feeling normal is not a reliable test.
Edibles and Urine Tests
Eaten marijuana follows a slower curve: lower peak blood THC, generally 2 to 4 hours after ingestion, with peak effects in the same window. A person can be least aware of the effect at the time they decide to drive.
Urine tests mostly detect THC-COOH. NHTSA reports that this inactive metabolite was detectable in plasma for days in controlled studies, and that a positive THC-COOH result "documents prior cannabis exposure but does not necessarily indicate impairment." Montana's per se limit excludes inactive metabolites.
What This Means for a Montana Case
For a per se charge, the State needs a properly drawn and analyzed sample showing active THC at or above the limit. For an impairment charge, a positive test alone is not enough; there must be other competent evidence that you were under the influence (MCA § 61-8-1018(1)(a)). In both, the questions are what was measured, when, by whom, and how the sample was handled. Our guide to challenging a blood test sets those out.
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, and it is not guidance on when it is safe to drive.