Legalizing cannabis for adult or medical use has never meant legalizing driving under its influence. Every US state that has legalized cannabis in any form still prosecutes driving under the influence of drugs (DUID), including cannabis, and penalties can be as severe as an alcohol DUI. But how each state defines "under the influence" for cannabis is inconsistent, scientifically contested, and very different from how alcohol impairment is measured.
Why Cannabis Doesn't Work Like Alcohol
The US has a single, nationally recognized standard for alcohol impairment: a blood alcohol concentration (BAC) of 0.08%, a level tied to decades of research showing a fairly direct relationship between blood alcohol and measurable driving impairment. Alcohol is water-soluble and clears the bloodstream within hours, so a blood test taken shortly after a stop reflects recent, active impairment.
THC behaves completely differently. It is fat-soluble, so it accumulates in fatty tissue and can be released back into the bloodstream well after the psychoactive effects have worn off — which is why frequent or daily cannabis users, including registered medical patients, can test positive for THC in their blood days or even weeks after their last use, long after any impairment has ended. There is no scientific consensus linking a specific blood THC concentration to a specific level of driving impairment, unlike with alcohol. A 2025 study out of UC San Diego found that heavy daily cannabis users performed normally on driving-relevant cognitive tests after 48 hours of abstinence, despite still having detectable THC in their blood.
How States Actually Set Their Limits
Because there is no reliable science behind a single number, states have taken very different legal approaches:
- Per se limits: a number of states have set a specific blood THC threshold above which a driver is automatically considered impaired under the law, regardless of other evidence. Pennsylvania uses 1 nanogram per milliliter (ng/mL), Ohio and Nevada use 2 ng/mL, and Montana and Washington use 5 ng/mL.
- Permissible inference: Colorado uses a 5 ng/mL threshold, but unlike a strict per se law, this only allows a jury to infer impairment — the defense can still present evidence that the driver was not actually impaired.
- Zero tolerance: some states criminalize driving with any detectable amount of THC or its metabolites in the body at all, with no minimum threshold.
- Impairment-based only: other states have no THC threshold in the law at all and instead require prosecutors to prove actual impairment through field sobriety tests, officer observation, or a Drug Recognition Expert (DRE) evaluation.
Zero-Tolerance States
Arizona and Illinois are commonly cited zero-tolerance states, where the underlying statute criminalizes driving with any detectable THC or metabolite in the body. In practice, both states have carved out partial protections: Arizona law specifies that a person cannot be convicted solely for having cannabis metabolites in their system unless they were impaired "to the slightest degree," and similar limits on metabolite-only prosecutions apply to registered medical patients in Illinois. Even so, the underlying zero-tolerance standard means visitors and residents alike face real legal exposure simply for having used cannabis recently, whether or not they are actually impaired at the time they are driving.
What This Means in Practice
The practical result is a legal patchwork that doesn't track how impairment actually works. A medical patient who used cannabis two days ago in a state with a per se or zero-tolerance law can fail a blood test while not being impaired at all, while an occasional user who consumed an hour before driving — genuinely impaired — might test under a per se threshold depending on their metabolism. Field sobriety tests and DRE evaluations are increasingly used specifically because blood THC numbers alone are recognized as unreliable evidence of impairment, but no state has an equivalent to the alcohol breathalyzer that reliably ties a roadside reading to real-time impairment.
For anyone driving in the US after using cannabis — resident or visitor, medical patient or recreational user — the only fully safe assumption is that any detectable THC carries legal risk somewhere in the country, regardless of how long ago it was used or how the driver actually feels.