Are There Roadside Cannabis Tests for DUI?

While marijuana is illegal under federal law, the state level may allow legal marijuana. As of 2026, 24 states have legalized recreational marijuana use, and 38 allow at least some form of medical marijuana, cannabis, or THC (tetrahydrocannabinol) use.

Yet, marijuana use presents a problem to traffic safety laws. Just as alcohol is legal but drinking and driving is not, driving while high is also illegal. Even if marijuana is legal in your state, it’s important to understand the current status of cannabis testing before you hit the road.

A drug DUI charge comes with the same severe consequences as impaired driving due to alcohol. In this article, learn how states measure impairment based on cannabis use — and the reliability of those tests.

But these laws vary significantly among states, and they change frequently. If you’re facing a drug DUI charge or another impaired driving charge, consult with a local DUI attorney as soon as possible. They can help you build a defense based on your state’s DUI and marijuana laws. A lawyer can protect your rights.

Driving Under the Influence of Marijuana

Most state laws consider “driving under the influence” to mean the influence of drugs as well as alcohol. Even prescription medications can result in DUI charges if they cause impairment. Any substances that cause drowsiness, dizziness, blurred vision, or slowed reaction time can impair your ability to drive.

But standard field sobriety tests were designed for alcohol impairment and may not be as reliable for detecting drug impairment.

Testing Alcohol Intoxication vs. Marijuana Impairment

When it comes to alcohol, state laws agree that if the average person reaches a blood alcohol concentration (BAC) of 0.08%, they are impaired and shouldn’t be driving.

However, unlike alcohol, there is no blood level of THC at which an average person becomes impaired. In a majority of states, there is no “acceptable” THC or blood-cannabinoid level for driving. Instead, some state legislators have turned to zero-tolerance or “per se” laws.

Zero-Tolerance Laws for Drugged Driving

Under a zero-tolerance law, any amount of drugs in a person’s system creates a presumption of impairment. Meanwhile, a per se law sets a threshold for presumed impairment, similar to the 0.08% BAC for alcohol.

Here’s a breakdown of states’ zero-tolerance and per se laws:

• Twelve states use a “zero tolerance” arrest policy. Any amount of THC (or a very small amount) in the blood is grounds for arrest for DUI or DWI.
• Five states (Illinois, Montana, Nevada, Ohio, and Washington) have per se laws. These states have established legal blood THC limits, and driving above these limits is a per se violation, even without signs of impairment.
• Colorado’s “permissible inference” law resembles the per se law, but allows a defendant to challenge the arrest with evidence that they were not impaired.

All other states require proof of impairment and a finding of THC in the defendant’s blood or urine. In other words, police stop drivers and test for proof of impairment through field sobriety tests, and then test for evidence of marijuana use.

Is There a Breathalyzer Test for THC?

No. Unlike alcohol, there are no reliable Breathalyzer or field sobriety tests for marijuana. There are alternative tests, but breath testing devices don’t measure marijuana use.

There are a number of chemical and biological reasons for this. Most involve how the body stores and metabolizes THC. The body stores THC in fat cells and slowly releases it into the bloodstream. Cannabis use also affects each person differently.

Decades of testing have shown that a person’s blood alcohol level directly correlates with their degree of impairment. Roadside breath tests are fairly reliable at providing a baseline reading for alcohol-impaired drivers.

Similar tests have not been able to prove this correlation with cannabis. Some users do show signs of impairment, but some do not.

Because of the way the body stores THC, it’s hard to tell the difference between:

• Active THC (which may indicate recent use/impairment)
• THC metabolites (which indicate past use but not necessarily impairment)

Tests can detect metabolites up to 30 days after use.

Roadside Marijuana Tests

Current testing methods only determine if a subject has used marijuana or cannabis. They don’t necessarily indicate whether the driver was impaired.

Some popular methods in use today include:

• Saliva tests: These tests use a strip or swab to measure THC levels in saliva. These oral fluid tests can detect marijuana use within about 24 to 48 hours.
• Blood tests: These tests are the most accurate and can detect THC in a person’s blood for several days (or even weeks for heavy users) after use. Blood tests often require a search warrant due to their invasiveness.
• Urine tests: Detects THC metabolites (THC-COOH), not necessarily active THC. Commonly used in workplace and sports settings, which can have zero-tolerance policies. Generally reliable for up to 30 days after use, but urine test results say nothing about impairment or recent use. However, frequency of use can also affect results. Occasional marijuana users may test positive for much shorter periods.
• Hair follicle tests: THC eventually gets deposited in hair follicles, providing a record of marijuana use as the hair grows. Tests can detect THC in hair cells for several months, depending on hair growth. Like urine testing, hair tests do not tell testers anything about current impairment.

Several states, including Indiana and Michigan, use Abbot Labs SoToxa mobile test system. This device uses an “oral fluid sample” (saliva) to test for the presence of half a dozen illegal drugs, including opiates, methamphetamines, and marijuana.

Roadside testing for cannabis is a rapidly evolving area of law. As research into the ways marijuana impacts drivers, some states are reconsidering their approaches to drug DUIs. 

Legal Consequences of Roadside Tests

All these tests, both roadside and lab tests, have an impact on your criminal justice case. The nature of the testing before or after your arrest can affect your DUI defense case.

All of the tests listed above can produce false positive and false negative results. Drunk driving tests for physical impairment do not always catch impairment due to drugs. The presence of drugs in hair or urine is not proof of present use or impairment.

Reasonable Suspicion of Driving While High

Law enforcement officers must have reasonable suspicion that someone is breaking the law to make a traffic stop. Then they must have probable cause to make an arrest for driving under the influence of alcohol or controlled substances.

In most states, a DUI arrest and conviction do not depend on your BAC or THC level. If a police officer observes someone driving erratically, this is typically enough reasonable suspicion for a DUI stop. Observed impairment is then probable cause for an arrest.

Challenging the Evidence

The roadside testing for marijuana impairment can also leave room to challenge the accuracy or reliability of the results.

Impairment can be proven through officer observations even without positive drug tests. Some agencies use drug recognition experts (DREs), officers who have received training in detecting the signs of narcotic impairment that differ from alcohol impairment.

However, it’s important to note that:

• Not all officers are drug recognition experts. This extensive program, developed by the National Highway Traffic Safety Administration (NHTSA) in conjunction with the International Association of Chiefs of Police (IACP), is a complex test that may or may not detect impairment.
• The DRE assessment is only the expert’s opinion, supported by the toxicological exam.
• DRE evaluations are typically conducted after police make an arrest, not during a traffic stop.

In most states, your attorney can challenge the results of a screening test and a field sobriety test. If your state has legalized marijuana use and you get arrested for driving under the influence, you should speak with a DUI attorney immediately.

Get Legal Advice from a Criminal Defense Attorney

Driving under the influence includes the influence of prescription medication and over-the-counter drugs, as well as alcohol and illegal narcotics. Police can pull you over for any erratic or illegal driving. Having a prescription will not get you off the hook. Even medical marijuana patients are not exempt from DUI laws.

If you get charged with a cannabis or marijuana DUI, you need legal advice from an experienced drug DUI defense lawyer. If you received any roadside drug testing, your attorney needs to know about it immediately to give you the best chance to challenge the charges.