Crimes » Marijuana Laws » DUI Marijuana

DUI Marijuana


The problemImpaired driving can lead to car accidents and people getting hurt.  If someone’s driving is impaired and if that was caused by marijuana, that may be a DUI marijuana offense.  And a conviction can result in all the DUI consequences, including driver’s license revocation.

Impaired driving

But when is a driver impaired?  Impairment means that our performance is less than normal to the point of being unsafe.  It could be less than normal for each one of us as an individual.  Or, we could mean less than normal for the law’s mythical “reasonable person.”

If we’re talking about impaired driving, then the impairment must connect with driving somehow.

So, before we get to DUI marijuana, we have the threshold question: Is the person’s driving impaired?

Bad driving

DUI marijuana: THC levels do not correlate with impairment
DUI marijuana: THC levels do not correlate with impairment

How can we tell if someone’s driving is impaired?  Well, is the person driving poorly?  Is it a “bad driving” case?

If a driver gets into a car accident, most wonder, was that driver at fault?  If not, a car accident does not show bad driving.  But it raises the question.

Or, a police officer may observe a driver driving badly.  Swerving across three lanes of traffic might mean bad driving.  Or, in isolation, the driver might have swerved to avoid hitting a deer.  Good driving.

Police officers attempt to observe and document more than one instance of bad driving, to help prove possible impairment.  This is the case in a DUI marijuana case as well.

Driver appears impaired

In some DUI cases, the police officer does not observe any driving.  For example, a person found near a car with a flat tire may arouse police suspicion.

Whether or not the police officer observes driving, they will try to get evidence to show drivers impairment.  If they find evidence of recent marijuana use, police may investigate for DUI marijuana.

“Field Sobriety Tests” or field exercises

DUI Marijuana: Don't perform requested Field Sobriety Tests like this one.
DUI Marijuana: Don’t perform requested Field Sobriety Tests like this one.

Police officers at a DUI stop ask the driver to perform “Field Sobriety Tests.”

The National Highway Traffic and Safety Administration (NHTSA) standardized them.

Common FSTs include the nine-step-walk-and-turn, the one-leg-stand and the Horizontal Gaze Nystagmus (HGN) test.

Police sometimes employ additional FSTs looking for evidence of DUI marijuana.  They may attempt to get the driver to cooperate with these at the police station.  And they may call in another officer to do their “Drug Recognition Evaluation.”

Police observations of driver

In addition, police will selectively document observations of the driver to support a theory of intoxication.  They ignore observations supporting a theory of sobriety.  Just about every DUI police report notes:

  • bloodshot, watery eyes
  • odor of (alcohol) (marijuana)

Other common notations include:

  • fumbled for license and insurance card
  • appeared nervous or tired
  • used door for balance exiting vehicle
  • used side of vehicle for balance

Preliminary Breath Test (PBT)

Most DUIs involve the odor of alcohol.  As a result, police officers often ask the driver to blow into a Preliminary Breath Test.  The PBT is a small, portable breath-alcohol machine.  Refusing can give the police arrest probable cause.  Blowing a 0.08 or more reading is also arrest probable cause.

In a traffic stop leading to a DUI marijuana investigation, police might request the driver blow into a PBT.  They will if the driver has an odor of alcohol.

Marijuana DUI specific procedures

For DUI marijuana investigations, the usual DUI investigation is a bit different.  Almost always, it begins with the police claim of an odor of marijuana.  Then, the police ask questions about marijuana.  Then, they search the driver and the car if they can. 

See How to Avoid a Marijuana Arrest in a Car in Minnesota.

If police find marijuana, they may charge marijuana in a motor vehicle over 1.4 grams.

Or, if they find wax or concentrates, they may charge a felony or gross misdemeanor possession.

After that, the FSTs or field exercises, become more important.  Many of them are the same as for alcohol.  But some FSTs relate specifically to drugs other than alcohol, including marijuana.

Know your rights: Never agree to do FSTs

Minneapolis Defense Lawyer Thomas Gallagher explains DUI marijuana
Minneapolis Defense Lawyer Thomas Gallagher explains DUI marijuana

Police cannot force anyone to do FSTs.  You can simply decline to do them.  You should not make any excuses, either.

Just tell the police officer that you won’t do any field exercises or Field Sobriety Tests.

NHTSA does not claim any FSTs are scientifically valid for marijuana.  The police officer is not a scientist or a medical doctor.  It’s not a fair “test.”  Police will note every imperfection to support one conclusion. “Fail.”  And they’ll ignore evidence to the contrary.  Don’t take that bait.

If you already did.  Your DUI marijuana attorney can take steps to neutralize them.


After an arrest at the roadside, the police officer takes the driver to either a hospital or a police station.  In order to support a DUI marijuana charge, they’ll need a urine or blood test.

Urine or blood test

To get a urine or blood test, police need a search warrant.  Ask to call an attorney prior to giving any sample for testing.

The marijuana DUI attorney on the phone can advise you about whether to submit a sample for testing; and about your right to an additional, independent test after theirs.

If you refuse a blood draw, police must offer the choice of a urine sample before labeling you a refusal.  And, they must offer a blood test if you refuse urine, before charging you with chemical test refusal.

Chemical test results

Police must send the blood or urine test to a lab for analysis.  That will take time.  Without the lab results, there is no evidence of marijuana or THC in the body.  So, charges must wait for the lab results, if any.

Lab reports for THC and metabolities

In a suspected DUI marijuana case, the lab will test for many prescription and other drugs, including THC.  In addition to THC, the lab report will show results for THC-metabolites.

What are metabolites?  When the body’s metabolism breaks down a chemical or drug into another chemical, that’s a metabolites.

Alcohol metabolites:  For example, enzymes in the human body metabolize alcohol first into acetaldehyde, a toxic substance. Then, in a second step, our bodies metabolize acetaldehyde down to another, byproduct called acetate.  Our bodies then break down Acetate into water and carbon dioxide for elimination.  Key take away:  Metabolites of alcohol are not alcohol.

THC and its metabolites
THC and its metabolites

Marijuana metabolites:  The only marijuana plant component that can cause impairment is THC.  So, the lab tests for the blood or urine for THC (Δ9-tetrahydrocannabinol).

In addition, the lab tests for two metabolites of THC, hydroxy THC (11-OH-THC)  and carboxy THC (THC-COOH).

Remember, a metabolite of THC is not THC.  Rather, a THC metabolite is another chemical.  The body makes it as a byproduct of breaking down THC into other, different chemicals.  And these two THC metabolites do not prove impairment.

Widespread ignorance about metabolites and their meaning

Ignorance about the science of THC, metabolites and their effects leads to false charges of DUI marijuana.  Attorney Gallagher has helped clients get these cases dismissed.

The common confusion about this may stem from the use of “THC” in THC metabolites, hydroxy THC, and carboxy THC.  So, remember:

  • THC is the psychoactive ingredient in marijuana, can cause impairment
  • THC metabolites are not THC, do not indicate current impairment

A lab test report includes levels for:

  • THC
  • Hydroxy THC, and
  • Carboxy THC

So, then why test for metabolites?  Because you can?  It’s difficult to discern any relevance of Carboxy THC for purposes of DUI enforcement.  After all, Carboxy THC:

  • has zero psychoactive effect; none
  • blood level rises after THC blood level declines
  • can remain detectable for over 30 days – so has zero value as proof of recent use
  • can prove past use, but not recent use or impairment

Hydroxy THC is detectable in blood up to 24 hours after last use, depending on method of ingestion.  As a result, we can use Hydroxy THC in combination with blood THC level to approximate time of last use.  Police and prosecutors sometimes do this in DUI marijuana cases.

Actual THC in blood does not prove impairment, regardless of level

“One of the program’s objectives was to determine whether it is possible to predict driving impairment by plasma concentrations of THC and/or its metabolite, THC-COOH, in single samples. The answer is very clear: it is not.

Plasma of drivers showing substantial impairment in these studies contained both high and low THC concentrations; and, drivers with high plasma concentrations showed substantial, but also no impairment, and even some improvement.”

US National Highway Traffic Safety Administration, Marijuana and Actual Driving Performance, 1993

NHTSA and the scientific studies say, there is no correlation between blood plasma THC levels and impaired driving.

That seems counterintuitive to most people.  Most people have personal experience with alcohol.  Blood alcohol levels do roughly correlate with impairment.  But alcohol and THC are quite different, in this way.

DUI marijuana: "subjects drive faster after drinking alcohol and slower after smoking marijuana"
DUI marijuana: “subjects drive faster after drinking alcohol and slower after smoking marijuana”

“THC’s effects differ qualitatively from many other drugs, especially alcohol. For example, subjects drive faster after drinking alcohol and slower after smoking marijuana. … Very importantly, our city driving studied showed that drivers who drank alcohol over-estimated their performance quality whereas those who smoked marijuana under-estimated it. …

“[S]ubjects in the marijuana group were not only aware of their intoxicated condition, but were … attempting to compensate for it. These … findings … support … the common belief that drivers become overconfident after drinking alcohol and … that they become more cautious and self-critical after consuming low doses of THC, as smoked marijuana.”

US National Highway Traffic Safety Administration, Marijuana and Actual Driving Performance, 1993

For more on the relative risks of DUI-marijuana, see our in depth article: The Myth of Marijuana DUI vs. Science

“… comparable to risks such as talking on a hands-free cellphone …”

The Minnesota Statutes

Minnesota Statutes §169A.20 is the general DUI statute.  Here are the most relevant parts:

“Subdivision 1Driving while impaired crime; motor vehicle.

It is a crime for any person to drive, operate, or be in physical control of any motor vehicle, as defined in section 169A.03, subdivision 15, except for motorboats in operation and off-road recreational vehicles, within this state or on any boundary water of this state when:

(1) the person is under the influence of alcohol;

(2) the person is under the influence of a controlled substance;

(3) the person is under the influence of an intoxicating substance and the person knows or has reason to know that the substance has the capacity to cause impairment;

(4) the person is under the influence of a combination of any two or more of the elements named in clauses (1) to (3); …


(7) the person’s body contains any amount of a controlled substance listed in Schedule I or II, or its metabolite, other than marijuana or tetrahydrocannabinols.

Subd. 2
Refusal to submit to chemical test crime.

It is a crime for any person to refuse to submit to a chemical test:

(1) of the person’s breath under section 169A.51 (chemical tests for intoxication), or 169A.52 (test refusal or failure; revocation of license); or

(2) of the person’s blood or urine as required by a search warrant under sections 171.177 and 626.04 to 626.18.

Penalties and consequences

The penalties and consequences of a marijuana DUI are similar to any Minnesota DUI case.

There is one important difference, however.

With an alcohol DUI, the state revokes your driver’s license immediately after a chemical test over the per se 0.08 limit.  And it revokes your license upon a refusal to test.  There is no per se limit for THC, however, and for good reason.

The science, and NHTSA agree, there is no specific THC level that correlates with impairment.  In fact, some drivers drive better with THC than other drivers with no THC.  As a result, a per se limit for THC would make no sense.

So, the state will not revoke your driver’s license before conviction for DUI marijuana, unless you refuse chemical testing.  Your attorney can help you with any questions about this.


The hundreds of defenses available in any DWI case are also available in a marijuana DUI.  But there are some defenses specific to DUI marijuana cases.  These include evidence about THC and its metabolites, and the effect of THC on driving.

Your DUI Marijuana Attorney

Should you have a DUI Marijuana Attorney with specific training and experience?  Attorney Thomas Gallagher has that.  In fact, Thomas Gallagher teaches DUI Marijuana law in Continuing Legal Education (CLE) courses to prosecutors, defense attorneys and judges.

Gallagher Criminal Defense logo sm

Thomas Gallagher is an expert DUI Marijuana defense attorney.

Would you like him to represent you?  Or, do you have a question?  You can call Attorney Thomas Gallagher for caring, expert assistance at 612 333-1500.