Drugged Driving and Marijuana DUI

Yes, DUI and DWI laws cover far more than alcohol. In every state, driving "under the influence" also includes being impaired by marijuana (even where it's legal medically or recreationally), prescription medications, and illegal drugs. Unlike alcohol, there is no scientifically reliable "per se" blood level that proves a person is impaired by THC (the active compound in marijuana) at the moment they were driving — which makes drugged-driving cases some of the most scientifically contested in criminal law, and the law in this area is still evolving state by state.

Why marijuana DUI cases are different from alcohol DUI cases

With alcohol, decades of research link blood alcohol concentration (BAC) to specific levels of impairment, which is why 0.08% is the per se legal limit in nearly every state (Utah is stricter, at 0.05%) — meaning a driver at or above that level is considered impaired as a matter of law, regardless of how they were actually driving.

THC does not work the same way. THC is fat-soluble, so it can linger in a person's blood and body for hours, days, or (in regular users) weeks after the impairing effects have worn off. That means a blood test showing THC in someone's system does not reliably show whether that person was impaired at the time they were driving — only that they used marijuana at some point recently. There is no consensus "impairment threshold" for THC the way there is for alcohol, and no roadside breath test currently in widespread law-enforcement use reliably measures THC impairment the way a breathalyzer measures alcohol.

Per se states vs. impairment states

Because the science is unsettled, states have taken different legal approaches to marijuana-related DUI:

• Per se limit states: Some states set a specific nanogram-per-milliliter THC threshold in blood; if a driver's test result is at or above that number, they can be charged as a matter of law, similar to the alcohol model — even without other evidence of bad driving.
• Zero-tolerance states: Some states make it illegal to drive with any detectable THC or its metabolites in the system, regardless of amount — a rule that can sweep in people who used marijuana legally days earlier and are no longer impaired.
• Impairment-based (effects) states: Other states do not use a THC number at all. Instead, the prosecution must prove actual impairment — through officer observations, field sobriety test performance, driving behavior, and sometimes expert testimony from a specially trained "drug recognition expert" officer.

Because these rules vary significantly and change frequently as marijuana laws evolve, confirm the current approach in your state or ask a local defense attorney — do not assume any specific number or rule applies to your case.

Prescription and over-the-counter medications count too

People are sometimes shocked to learn they can be charged with DUI/DWI for taking medication exactly as prescribed — sedatives, opioid painkillers, anti-anxiety medications, and even some allergy or sleep medications can support a drugged-driving charge if they actually impaired the driver's ability to operate a vehicle safely. Having a valid prescription is not an automatic defense; the legal question is whether the medication impaired the driver, not whether the drug itself was legal to possess.

How drug DUI cases are investigated and tested

Because standard roadside breathalyzers only measure alcohol, drug-DUI cases typically rely on:

• Field sobriety tests — the same balance and coordination tests used in alcohol cases, though their reliability for detecting drug impairment (as opposed to alcohol impairment) is debated.
• Drug Recognition Expert (DRE) evaluations — specially trained officers conduct a multi-step examination (vital signs, pupil size, muscle tone, etc.) to form an opinion about impairment and drug category.
• Blood tests — the primary chemical evidence in drug cases, since blood can be tested for a broad panel of substances that breath tests cannot detect. Urine testing is sometimes used but is even less useful for pinpointing timing of impairment.

Blood draws implicate real constitutional protections. In Birchfield v. North Dakota (2016), the U.S. Supreme Court held that the Fourth Amendment generally requires a warrant (or a recognized exception, such as voluntary consent or exigent circumstances) before police can force a blood draw — unlike a breath test, which the Court treated differently. If your blood was drawn without a warrant, without your voluntary consent, and without another valid legal basis, that is a serious issue your lawyer should examine, since illegally obtained evidence can potentially be challenged and excluded (a principle rooted in Mapp v. Ohio, 1961, which applied the Fourth Amendment's exclusionary rule to the states).

Sobriety checkpoints themselves — where police stop vehicles systematically to check for impairment — have been upheld as constitutional under limited circumstances by the Supreme Court in Michigan Dept. of State Police v. Sitz (1990), so the existence of a checkpoint is not, by itself, a valid defense; the details of how it was conducted may still matter.

Your core rights in any drugged-driving stop

• You are presumed innocent, and the prosecution must prove every element of the charge beyond a reasonable doubt — you do not have to prove your innocence.
• You have the right to remain silent and the right to an attorney, including during police questioning. If you are in custody and being interrogated, police must give Miranda warnings (Miranda v. Arizona, 1966) before that questioning can be used against you; politely stating that you wish to remain silent and want a lawyer is enough to invoke these rights.
• You have the right to counsel at every critical stage of a criminal case, and if you cannot afford a lawyer, one must be appointed for you (Gideon v. Wainwright, 1963).
• The Fourth Amendment protects you against unreasonable searches and seizures, including certain vehicle stops and blood draws; brief investigative stops require reasonable suspicion (Terry v. Ohio, 1968), and prosecutors must turn over material evidence favorable to the defense (Brady v. Maryland, 1963).
• You have the right to effective assistance of counsel (Strickland v. Washington, 1984), the right to represent yourself if you choose (Faretta v. California, 1975), and the right to a speedy trial (Barker v. Wingo, 1972).

What to do if you're facing a drugged-driving charge

1. Stay calm and polite, but exercise your right to remain silent about how much you used, when, or how you feel — anything you say can be used as evidence of impairment.
2. Do not resist or obstruct testing procedures; instead, ask whether you are legally required to submit to a specific test under your state's "implied consent" law, and understand that refusing a test can carry its own separate consequences (often an automatic license suspension) independent of the criminal case. Never attempt to evade or physically resist officers — raise legal objections in court, not at the roadside.
3. Check for a DMV/license hearing deadline immediately. Many states have a separate administrative process tied to license suspension after a DUI arrest or test refusal, with a very short window — sometimes just days — to request a hearing to keep your driving privileges. Missing this deadline can mean automatic suspension even if you are never convicted. Confirm your state's specific deadline right away, because this is easy to miss and hard to undo later.
4. Write down what happened while it's fresh — the reason given for the stop, field sobriety instructions, whether you were read your rights, and the timeline of any blood draw or test.
5. Contact a criminal defense attorney promptly. Drugged-driving cases often turn on technical, scientific issues — chain of custody for blood samples, lab testing methods, DRE officer qualifications, and constitutional challenges to the stop or the blood draw — that benefit from experienced legal review.
6. Do not skip any court date without talking to a lawyer first; missing court can add separate charges on top of the DUI itself.

The law is still evolving

As more states legalize or decriminalize marijuana, legislatures and courts are actively revisiting how to fairly measure drug impairment behind the wheel. Some states have moved away from rigid per se THC limits toward impairment-based standards precisely because of the science described above. This is an area where the rules in your state may have changed recently or may change again — always confirm current law rather than relying on older information, including this article.

Key takeaways

• DUI/DWI laws apply to marijuana (legal or not) and to prescription and over-the-counter medications, not just alcohol.
• There is no reliable scientific way to translate a THC blood level into "impaired at the time of driving" the way BAC works for alcohol.
• States differ sharply: some use a per se THC number, some use zero-tolerance rules, and some require proof of actual impairment.
• Blood draws generally require a warrant, valid consent, or another recognized exception under Birchfield v. North Dakota.
• If your license is at risk through an administrative/DMV process, the deadline to request a hearing can be extremely short — act immediately and confirm your state's rule.

This article is general legal information, not legal advice, and reading it does not create an attorney-client relationship. If you are facing a drugged-driving charge, talk to a licensed criminal defense attorney in your state as soon as possible.