The short answer
In a zero tolerance state, a DUI charge can rest on a blood test alone. If THC or a THC metabolite shows up, the state has its case. Prosecutors do not need to show that the driver swerved, sped, or failed a field sobriety test. More than 20 states use this standard or a close version of it. The list changes as legislatures act, so check the statute for the state where you drive.
Is Driving on Medical Marijuana Legal?
States on the zero tolerance list
State highway safety offices and the Insurance Institute for Highway Safety track these laws. Common lists name these states:
What Happens if You Get a DUI for Weed
- Arizona
- Delaware
- Georgia
- Idaho
- Illinois
- Indiana
- Iowa
- Kansas
- Kentucky
- Michigan
- Minnesota
- North Carolina
- Oklahoma
- Rhode Island
- South Dakota
- Tennessee
- Virginia
- West Virginia
- Wisconsin
That is 19 states. Some trackers add one or two more. Several states that allow medical or adult-use cannabis keep a zero tolerance driving rule. Illinois and Virginia are two examples.
States with a number instead of zero
Five states set a per se THC limit in blood:
zero tolerance marijuana driving states
- Colorado: 5 ng/mL of active THC
- Montana: 5 ng/mL
- Nevada: 2 ng/mL
- Pennsylvania: 5 ng/mL
- Utah: 5 ng/mL
A number works much like zero in court. The state shows the lab result and the case moves forward. Colorado lets the driver argue that the result did not equal impairment.
Why the metabolite rule matters
Labs look for two compounds. Delta-9 THC is the active drug. Carboxy-THC, also called COOH-THC, is an inactive metabolite. Carboxy-THC can stay in blood for a day or more and in urine for weeks. Impairment fades in hours. A zero tolerance law that counts any metabolite can reach a driver who used cannabis days before the stop and was not high at the wheel.
Court rulings that narrowed the rule
Two decisions changed practice in two states. In People v. Koon (Michigan, 2023), the state supreme court held that carboxy-THC in blood alone does not show impairment. In Commonwealth v. Barr (Pennsylvania, 2020), the court ruled that a medical cannabis patient cannot be convicted on a carboxy-THC result alone. Read your state's case law, not just the statute.
What to check
- Find the state DUI statute and look for the phrase "any amount" or "any detectable amount."
- Look for a nanogram per milliliter number.
- Note which compound the law names, active THC or metabolite.
- Check for a medical cannabis defense.
Legal cannabis in your state does not make driving legal. No blood THC level maps to a fixed degree of impairment. The safest rule is to not drive after use.