There is no single legal limit for THC while driving in the United States. Unlike the .08 blood alcohol standard that applies everywhere, the THC threshold depends on your state: some set a specific blood concentration between 1 and 5 nanograms per milliliter, some ban any detectable trace, and most require a prosecutor to prove you were actually impaired. The same amount of cannabis in your blood can be perfectly legal in one state and a DUI a few miles across the border.
Why There Is No Single THC Number
Alcohol is water-soluble. It spreads evenly through the bloodstream and clears at a predictable rate, so a blood or breath number tracks impairment reasonably well across most people. THC does not behave that way. It is fat-soluble, absorbed quickly into fatty tissue after consumption, and then released back into the blood over hours, days, or weeks.
A National Highway Traffic Safety Administration report to Congress found that peak impairment does not line up with peak THC blood levels. You can test high while feeling normal, and you can test low while meaningfully impaired. NHTSA concluded that THC concentration in blood and the degree of impairment “do not appear to be closely related.”1National Highway Traffic Safety Administration. Marijuana-Impaired Driving: A Report to Congress
Tolerance makes it worse. A daily user may carry low but detectable THC for weeks after last consuming, with no impairment at all. A first-time user with the same blood level could be noticeably impaired. That scientific reality is why states have landed on four very different frameworks instead of one shared number.
The Four State Frameworks
Which framework your state uses decides what evidence can convict you, and whether the number in your blood is the whole case or just one piece of it.
Per Se Limits
About half a dozen states set a specific THC blood concentration that works like the .08 line for alcohol. If you are at or above it, you are legally impaired, no matter how you drove or how you performed on field tests. These per se limits run from 1 to 5 ng/mL of blood.2National Highway Traffic Safety Administration. Drug-Impaired-Driving Laws The number alone is enough.
Zero-Tolerance Laws
Roughly a dozen states make it illegal to drive with any measurable amount of THC or its metabolites in your body.3Governors Highway Safety Association. Drug-Impaired Driving Because metabolites can linger for days or weeks after effects wear off, these laws can reach someone who used cannabis days earlier and is completely sober at the wheel. In states that have also legalized recreational or medical cannabis, courts have started requiring prosecutors to show actual impairment alongside the positive test.
Permissible Inference Laws
At least one state uses a middle ground. If your blood THC exceeds 5 ng/mL, a jury is allowed to infer that you were impaired, but the inference is not automatic. You can rebut it with evidence like regular-user tolerance or normal driving behavior.2National Highway Traffic Safety Administration. Drug-Impaired-Driving Laws A per se limit is conclusive; a permissible inference is a starting point the defense can challenge.
Impairment-Based Laws
Most states use this approach and set no specific THC number at all.4National Conference of State Legislatures. Drugged Driving: Marijuana-Impaired Driving The prosecutor has to prove marijuana actually impaired your driving. Evidence typically comes from the officer’s observations of your driving, your appearance, your performance on field sobriety tests, and any chemical test, which corroborates rather than stands alone. These cases are harder for the state to win and lean heavily on the arresting officer’s judgment.
How the Number in Your Blood Gets Measured
Cannabis impairment is harder to detect at the roadside than alcohol, because there is no quick breath device for THC. If an officer suspects drug impairment, they may call in a Drug Recognition Expert, a specially trained officer who runs a standardized evaluation looking for signs like elevated pulse, dilated pupils, lack of eye convergence, and poor performance on divided-attention tasks.5National Highway Traffic Safety Administration. Preliminary Training for the Drug Evaluation and Classification Program – Participant Manual
Blood tests are the standard for measuring active THC and are considered the most reliable indicator of recent use. Active THC typically shows up in blood for a few hours after consumption, though heavy, chronic users can test positive for days or weeks because of how the body stores and slowly releases it.1National Highway Traffic Safety Administration. Marijuana-Impaired Driving: A Report to Congress Blood draws usually happen at a hospital or station after arrest, not on the roadside.
Oral fluid tests, collected with a mouth swab, are increasingly used for roadside screening. Detection windows run from roughly two hours to more than 30 hours depending on the person and the cutoff used.6AAA Foundation for Traffic Safety. Detection Windows for Drugs in Oral Fluid: Cannabinoids, Stimulants, and Opioids Urine tests detect metabolites rather than active THC. Those metabolites can persist for weeks after last use, so urine results cannot establish current impairment and are mainly useful in zero-tolerance states where any detectable amount is enough.
Can You Refuse the Test
Every state has an implied consent law. By driving on public roads, you have already agreed, as a condition of your license, to submit to chemical testing when an officer has probable cause to suspect impairment. Refusing a test does not make the case go away. In most states, refusal triggers an automatic administrative license suspension through the motor vehicle agency, immediate and separate from any criminal DUI penalty, and it applies even if you are never convicted.
A blood draw is a much more invasive search than a breath test, and the U.S. Supreme Court has drawn a line there. Officers may administer a breath test without a warrant after a lawful DUI arrest, but they generally cannot force a blood draw without a warrant or a recognized exception like genuine exigent circumstances. The Court also held that states cannot criminalize refusing a blood test based on implied consent alone.7Justia Law. Birchfield v. North Dakota, 579 U.S. ___ (2016) And the fact that THC naturally declines in the bloodstream over time does not automatically justify skipping the warrant, because the Court rejected a blanket exigent-circumstances exception for metabolizing substances.8Justia Law. Missouri v. McNeely, 569 U.S. 141 (2013)
So refusing a blood test may give your attorney an argument about the evidence, but it almost certainly costs you your license for an extended period through the administrative process. Repeat refusals carry escalating suspensions and fines in many states.
Commercial Drivers Face Stricter Rules
If you hold a commercial driver’s license, the state frameworks above do not help you. Federal regulations classify marijuana as a Schedule I controlled substance, and a CDL holder is not physically qualified to drive a commercial vehicle if they use it. This applies regardless of whether your state has legalized recreational or medical cannabis.9Federal Motor Carrier Safety Administration. Drug and Alcohol Testing FAQs You also cannot possess marijuana or be under its influence while on duty.
DOT-regulated drug testing uses urine screens with an initial cutoff of 50 ng/mL for marijuana metabolites and a confirmatory cutoff of 15 ng/mL.10eCFR. 49 CFR 40.85 – What Are the Cutoff Concentrations for Drug Tests Those tests detect metabolites, not active THC, so use days or weeks before the test can still produce a positive result. Testing happens at hiring, randomly during employment, after accidents, and whenever an employer has reasonable suspicion. A positive test or refusal triggers immediate removal from safety-sensitive duties and is recorded in the FMCSA Drug and Alcohol Clearinghouse, where future employers can see it.11Drug and Alcohol Clearinghouse. Violations and Return-to-Duty FAQs
A Medical Card Does Not Change the Limit
A medical marijuana card does not protect you from a DUI charge. The right to use cannabis for a medical condition is separate from the right to drive, and every state’s impaired driving laws apply to patients the same way they apply to recreational users. If your blood exceeds a per se limit, or if a prosecutor can show your driving was impaired, the fact that a doctor recommended the cannabis is irrelevant to the DUI case.
Most states also do not require employers to accommodate medical cannabis use, and employers generally retain the right to enforce drug-free workplace policies.12National Conference of State Legislatures. Cannabis and Employment: Medical and Recreational Policies in the States The assumption that a card provides a defense is one of the most common and costly mistakes drivers make in this area of the law.