THC blood tests and marijuana impairment in Michigan OWI cases

A Michigan blood test for marijuana can report two very different things: active THC (delta-9-THC, the substance that affects the brain) and THC metabolites, which are byproducts the body makes as it breaks THC down.^[2] The difference matters because the Michigan Supreme Court held in People v Feezel (2010) that the inactive metabolite 11-carboxy-THC "is not a schedule 1 controlled substance."^[2] Michigan law also sets no numeric THC limit for drivers.^[1]

This page explains what a THC result means in each type of OWI charge, what Michigan's appellate courts have said, and what the state's Impaired Driving Safety Commission found about THC numbers and impairment. The broader rules are in the marijuana and driving law guide.

Three ways a marijuana case can be charged

Charge	What must be shown about marijuana
OWI, "under the influence" (MCL 257.625(1)(a))	The driver was under the influence of a controlled substance or other intoxicating substance.[6][1]
Operating while visibly impaired (MCL 257.625(3))	The driver was visibly impaired by a controlled substance or other intoxicating substance; Dupre held the Medical Marihuana Act does not supersede the OWVI statute.[1][7]
Any amount of a Schedule 1 drug (MCL 257.625(8))	Any amount of a Schedule 1 controlled substance in the body while operating, whether or not the driver was impaired.[8]

For the first two charges, a THC number is one piece of evidence about impairment. For the third, the question is whether the substance found is a Schedule 1 substance at all. Penalties for 625(8) are the same as for OWI under 625(9).^[8] When an OWI or OWVI charge rests on a drug, the jury or judge must also make a written finding on whether the person was under the influence of or impaired by the drug, which the court reports to the Secretary of State.^[9]

Active THC versus metabolites

Feezel: the inactive metabolite does not count

Michigan's Public Health Code lists marihuana in Schedule 1.^[3] In Feezel, the Supreme Court held that 11-carboxy-THC, "a byproduct of metabolism created when the body breaks down the psychoactive ingredient of marijuana," is not a Schedule 1 controlled substance "because the Legislature did not intend for it to be a schedule 1 controlled substance under MCL 333.7212."^[2] The decision overruled People v Derror (2006) on that point.^[2] The result is that the presence of 11-carboxy-THC alone does not prove the 625(8) offense.^[10]

The same logic for other drugs

In People v Stock (2021), the Supreme Court reversed OWI-causing-death and serious-impairment convictions where the prosecution showed only an unidentified cocaine metabolite in urine, without evidence that cocaine itself was in the defendant's body.^[11] The court rejected a reading of "controlled substance" in 625(8) "that would include any metabolite of cocaine."^[11]

Active THC still counts under 625(8)

Marijuana, including pharmaceutical-grade cannabis, remains on Michigan's Schedule 1 except when handled in compliance with the Public Health Code and as authorized by federal authority.^[3] The Traffic Benchbook says 625(8) "continues to prohibit the operation of a vehicle if the operator has any amount of a schedule 1 substance, which includes marihuana."^[4] In People v Perry (2021), an 18-year-old whose blood showed 4 nanograms of THC per milliliter after a crash could be prosecuted under 625(8).^[12]

Medical and recreational marijuana users

• Registered medical patients. Koon (2013) held that the Medical Marihuana Act supersedes 625(8) for a registered patient who has marijuana in his or her system "but is not otherwise under the influence of marijuana."^[13] The Act itself does not authorize driving under the influence.^[14] Dupre (2020) held it does not shield patients from OWVI prosecution.^[7]
• Adult recreational users. The recreational law, effective December 6, 2018, does not authorize driving under the influence of marijuana, consuming it while driving, or smoking it in the passenger area of a vehicle on a public way.^[15] The Traffic Benchbook says it is "unclear if the Koon holding … will be extended to the MRTMA."^[4]
• Drivers under 21. Perry held the recreational law "did not remove all criminal penalties for persons under the age of 21 who operate a motor vehicle with marijuana in their system."^[12]

Why a THC number does not show impairment the way a BAC does

For alcohol, Michigan sets per se levels of 0.08 and 0.17.^[6] There is no matching THC level.^[1] The Legislature created the Impaired Driving Safety Commission in 2016 PA 350 to research and recommend a scientifically supported THC threshold for per se impaired driving.^[16] Its March 2019 report reached these conclusions:

• "There is no scientifically supported threshold of Δ9-THC bodily content that would be indicative of impaired driving," because driving impairment correlates poorly with blood (plasma) THC at the time of collection.^[5]
• THC is first eliminated very rapidly, so levels "may be very low by the time blood is drawn," which could underestimate the level at the time of driving.^[5]
• In long-term users, blood THC "can remain above 1 ng/ml for 48-72 hours," so the Commission said Michigan's "no tolerance" policy "might falsely conclude that an individual is impaired."^[5]
• There is "strong scientific evidence" that tolerance develops with regular and frequent use, so infrequent users may be impaired at lower blood THC levels than regular users.^[5]
• The Commission "recommends against the establishment of a threshold" and recommends roadside sobriety tests instead.^[5]

The Commission's report is a policy recommendation, not a court ruling. The Vehicle Code was not changed to add a THC limit; this guide found no statute adding one.^[1]

How the blood is tested and reported

When blood is drawn at a police request, only a licensed physician or a qualified person under a physician's delegation, in a medical environment, may draw it.^[17] State Police rules require that the skin not be cleaned with alcohol, that blood be drawn into a tube with sodium fluoride preservative, and that collection be witnessed, labeled and sealed.^[18] Accepted methods for drugs include LC/MS or GC/MS and immunoassays, with daily control samples kept at least 1 year.^[19]

The Michigan State Police Toxicology Unit serves the whole state from its Lansing Laboratory and analyzes about 5,500 cases a year for drugs.^[20] The detail of blood-draw rules is in the blood tests guide.

Saliva numbers are not blood numbers

The State Police oral-fluid pilot report warned that THC levels in oral fluid differ greatly from blood, citing a study that equated about 1 ng/mL in blood to about 44 ng/mL in oral fluid.^[21] See the roadside saliva testing guide.

Other evidence of marijuana impairment

A THC result is not the only possible evidence in a drug case. The chemical-testing statute does not limit other admissible evidence about impairment.^[22] Drug recognition expert opinions have their own limits: in Bowden (2022), the Court of Appeals reversed an order letting a DRE testify that a driver was impaired by marijuana to an unsafe degree, because the prosecution did not show the DRE protocol was validated for that purpose.^[23] See the DRE guide and the drug-impaired driving evidence guide.

Common misunderstandings

Belief	What the law says
"Michigan's THC limit is 5 nanograms."	The Vehicle Code has no numeric THC limit.[1]
"Marijuana is legal, so THC in my blood is fine."	Marijuana remains in Schedule 1 and 625(8) still bars any amount, subject to Koon for medical patients and an unsettled question for recreational users.[3][13][4]
"Any THC-related result means a conviction."	The metabolite 11-carboxy-THC alone is not a Schedule 1 substance under Feezel.[2]
"Under 21 is covered by the recreational law."	Perry held the recreational law did not remove 625(8) penalties for under-21 drivers.[12]

Drivers under 21 face additional rules; see the under-21 zero tolerance guide.