Legal Limit for Driving High: Per Se, Zero Tolerance, and Penalties

There is no single national legal limit for driving high on marijuana. Five states set a specific blood-THC number, twelve states treat any detectable THC as a crime, and the rest require prosecutors to prove you were actually impaired without pointing to a number.¹ Where a numeric limit exists, 5 nanograms of THC per milliliter of whole blood is the most common cutoff, and one state sets it as low as 2 nanograms. Which rule applies to you depends entirely on where you’re driving.

Why a THC Number Isn’t Like a BAC Number

Alcohol moves through the body predictably. Blood-alcohol rises as you drink, peaks within about 20 minutes of your last drink, then falls at a roughly steady rate, and impairment tracks that curve closely. THC behaves nothing like that.²

THC is fat-soluble. After you consume marijuana, it stores in fatty tissue and can re-enter the bloodstream long after the high is gone. Blood levels spike quickly, then taper off at an uneven rate, and peak impairment often doesn’t line up with peak blood concentration. NHTSA’s own report to Congress states that “the level of THC in the blood and the degree of impairment do not appear to be closely related” and that blood THC is “not an accurate and reliable predictor of impairment.”²

The practical result is unfair in both directions. A frequent user can register above a per se limit days after their last use, feeling completely sober. An occasional user can be meaningfully impaired at a level below the legal cutoff. Research confirms that THC and its main metabolite behave very differently in daily versus occasional users, which makes a single numeric standard unreliable as applied to any individual driver.³ That scientific uncertainty is why states have landed in three different places.

States With a Per Se THC Blood Limit

Five states have per se laws that fix a THC blood concentration as the legal line. Illinois, Montana, Ohio, and Washington use 5 nanograms per milliliter. Nevada sets its felony threshold at 2 nanograms per milliliter.¹ If the blood test comes back at or above the limit, you are legally impaired as a matter of law. How you were actually driving does not factor in.

Colorado uses a related but softer rule called a “permissible inference.” At 5 nanograms or more, the jury is allowed to presume you were impaired, but you can push back with evidence: a daily medical-use baseline, clean driving, anything that makes the presumption less convincing. A pure per se state offers no such opening.¹

Zero-Tolerance States

Twelve states go further. Any detectable amount of THC or its metabolite in your blood while driving is a crime. Those states are Arizona, Delaware, Georgia, Indiana, Iowa, Michigan, Oklahoma, Pennsylvania, Rhode Island, South Dakota, Utah, and Wisconsin.¹

The word “metabolite” is what gives these laws their reach. Metabolites are the chemical byproducts left as your body breaks down THC. Active THC may clear in hours, but the metabolite THC-COOH can linger in regular users for days or weeks. In a zero-tolerance state, you can be charged well after any impairment has passed. You do not need to feel high, look high, or drive poorly. The chemistry alone is enough.

States That Require Proof of Actual Impairment

The remaining states don’t use a blood-THC number at all. Prosecutors must prove your driving ability was actually diminished by marijuana. Evidence comes mostly from the arresting officer: bloodshot eyes, slurred or slowed speech, the smell of marijuana, poor coordination, weaving, delayed reactions. These observations are subjective, which makes the cases harder to prosecute and harder to defend. There is no number to argue over, so the trial often turns on whether the jury believes the officer.

To firm up these cases, departments increasingly rely on Drug Recognition Experts: officers trained through a joint NHTSA and International Association of Chiefs of Police program to run a structured 12-step evaluation and identify the category of drug involved.⁴ A DRE’s opinion still rests on human judgment, and courts have split on how much weight to give it.

How Testing Works, and What Refusal Costs

After a stop, the chemical evidence typically comes from a blood draw, a urine sample, or an oral fluid swab. Blood is the standard for measuring active THC and is the main evidence used in court. Urine detects metabolites but says little about recent use. Saliva testing is newer and increasingly used at the roadside, where devices can indicate THC presence within about 15 minutes. A positive roadside saliva screen is preliminary. It gives the officer probable cause for arrest and a lab test, but on its own it usually isn’t enough to convict.⁵

Every state has an implied consent law. By holding a driver’s license, you’ve already agreed in advance to chemical testing when an officer has reasonable grounds to suspect impairment. Refusal triggers automatic administrative penalties, most commonly a one-year license suspension for a first refusal and longer for later ones. In some states refusal is itself a separate crime.

One constitutional limit matters here. In Birchfield v. North Dakota (2016), the U.S. Supreme Court held that while states can require a warrantless breath test as part of a lawful DUI arrest, they cannot impose criminal penalties for refusing a warrantless blood draw. The Court said drivers cannot be “deemed to have consented to submit to a blood test on pain of committing a criminal offense.”⁶ In practice, officers who want blood from an unwilling driver generally need a warrant from a judge.

Penalties for a First Offense

Penalties vary by state, but the shape of a first marijuana DUI sentence looks similar most places:

Jail time ranging from 24 hours to one year, with many states imposing a mandatory minimum of one or two days. Some states let judges suspend most of the jail term if you complete a treatment program.
Base fines of roughly $250 to $2,000, before surcharges, court fees, and mandatory assessments, which routinely push the real total much higher.
A license suspension of 90 days to one year, sometimes with a restricted or hardship license available for part of that period.
A substance abuse evaluation, and often completion of an education or treatment program as a condition of sentencing or license reinstatement.

A first-offense marijuana DUI is usually a misdemeanor, which still creates a criminal record. That record appears on background checks and can affect employment, housing, professional licensing, and immigration status. In many states the conviction stays on your driving record permanently even if the criminal case is eventually eligible for expungement.

The Longer Financial Hit

Court fines are only the opening cost. Most states require convicted drivers to file an SR-22, a certificate proving you carry at least the state-minimum liability insurance, and you typically must keep it in place for three years. The insurers willing to issue one charge noticeably more.

After a DUI conviction, premiums commonly rise 50 to 150 percent, and some drivers see steeper jumps depending on prior record and state. The increase isn’t one-time. It follows you for three to seven years, and some insurers factor a DUI into your rate for a full decade. Add license reinstatement fees of roughly $100 to $200, court-ordered substance abuse programs, possible ignition interlock installation, and towing or impound charges from the night of the arrest. The total out-of-pocket cost of a first-offense DUI routinely exceeds $10,000 once everything is added up.

Commercial Drivers Live Under Federal Rules

If you hold a commercial driver’s license, state law isn’t the controlling authority. The Department of Transportation requires drug testing for all safety-sensitive transportation employees under 49 CFR Part 40, and marijuana sits on the mandatory five-drug panel.⁷ Under 49 CFR Part 382, no CDL holder may report for duty or stay on duty while using any Schedule I substance, which includes marijuana no matter what your state law says.⁸

A positive DOT test doesn’t just suspend your CDL. It immediately removes you from all safety-sensitive functions, and you cannot return until you’re evaluated by a Substance Abuse Professional, complete the recommended treatment, and pass a return-to-duty test. Follow-up testing continues for years afterward.⁷

As of early 2026, marijuana remains a Schedule I controlled substance for federal purposes despite ongoing rescheduling discussions. DOT and FMCSA have stated that their testing requirements and zero-tolerance posture on marijuana are unchanged and will stay that way until any rescheduling is complete.⁹ A commercial driver who uses marijuana in a state where it’s recreationally legal is still violating federal regulations and risking their livelihood.

A Medical Marijuana Card Won’t Help

A valid medical marijuana recommendation doesn’t exempt you from drugged driving laws. In per se and zero-tolerance states, the charge is based on what’s in your blood, not why it’s there. A prescription doesn’t change the chemistry. In impairment-based states, the question is whether your driving was affected, and a medical card doesn’t erase observed impairment.

A handful of states technically allow medical authorization to be raised as an affirmative defense, but winning on that basis is rare. Most defense attorneys treat a medical card as useful context for sentencing negotiations rather than a viable trial defense. The safest assumption in every state is that your medical authorization offers no legal protection once you’re behind the wheel.

1
National Conference of State Legislatures. Drugged Driving | Marijuana-Impaired Driving
2
National Highway Traffic Safety Administration. Marijuana-Impaired Driving: A Report to Congress
3
National Library of Medicine. Identification of Recent Cannabis Use: Whole-Blood and Plasma Free and Glucuronidated Cannabinoid Pharmacokinetics Following Controlled Smoked Cannabis Administration
4
National Highway Traffic Safety Administration. Drug Evaluation and Classification Program Participant Manual
5
National Conference of State Legislatures. States Explore Oral Fluid Testing to Combat Impaired Driving
6
Justia. Birchfield v North Dakota, 579 US ___ (2016)
7
eCFR. 49 CFR Part 40 – Procedures for Transportation Workplace Drug and Alcohol Testing Programs
8
eCFR. 49 CFR Part 382 – Controlled Substances and Alcohol Use and Testing
9
FMCSA Drug and Alcohol Clearinghouse. In Case You Missed It: Updates from ODAPC