While researchers are currently trying to create a workable breathalyzer-type device to test for THC, or the psychoactive compound in marijuana, modern breathalyzers cannot detect THC or weed.
Rather, police officers currently rely on field sobriety tests and tests that examine saliva and blood to help determine if you are under the influence of marijuana. Law enforcement may also rely on a drug recognition expert (DRE).
Note that while many states have legalized the recreational and medicinal use of marijuana, it is still a criminal offense to drive while high on cannabis.
Most states say that you are guilty of driving under the influence of drugs (DUID) if:
- you drove a vehicle, and
- while you drove, you were under the influence of a controlled substance.
If guilty of DUID, you could face some of the following penalties:
- a misdemeanor conviction,
- jail time,
- fines,
- a driver’s license suspension, and
- completion of a DUI school.
1. Can a breathalyzer detect marijuana?
Current breathalyzer tests cannot detect THC (or tetrahydrocannabinol) or cannabis use. The same holds true for ignition interlock devices (IIDs). But researchers are trying to create a roadside testing device that police can confidently use to measure levels of THC in a person’s breath sample.
For example, researchers at the University of Pittsburgh have successfully created a marijuana breathalyzer prototype that can detect THC levels in a breath test. The researchers, led by Alexander Star, PhD, created the new breathalyzer by using carbon nanotubes, or tiny tubes of carbon that are 100,000 times smaller than a human hair.1
Please note, though, that even if a breathalyzer could detect THC, there is currently no acceptable cut-off limit for THC impairment. For example, most states say that you are guilty of drunk driving (or DUI) if alcohol breathalyzers show a blood alcohol content (BAC) of .08%. Though to the disappointment of many lawmakers, states have not settled on a similar quantifiable number for marijuana use THC impairment.
2. How do police determine if I am under the influence of marijuana?
Police officers first try to detect marijuana impairment via observations and field sobriety tests.
A few signs that you are under the influence of marijuana, or controlled substances include:
- slurred speech,
- poor balance,
- slow motor skills,
- bloodshot eyes, and
- impaired cognitive function.
Most states then say that if officers have probable cause to believe you are under the influence from smoking marijuana (or using edibles), they can ask you to submit to a:
- saliva test,
- blood test, or
- urine tests.
Note, though, you can refuse these tests. The most common refusal is a blood test refusal.
Most states say that a blood test refusal is illegal and can lead to an automatic driver’s license suspension for up to one year.2
Though while a refusal is illegal, most states say that there are certain situations where police can require you to submit to a blood test (for example, when police have a warrant for the test).
3. What about drug recognition experts?
Authorities can use a drug recognition expert (DRE) to help determine if you are driving while high.
A DRE is a law enforcement officer with special training. The special training helps the officer identify when you are under the influence of a controlled substance.
Note that not all counties have DREs. So, if you are suspected of DUID, you may or may not be subject to a DRE evaluation.
4. When can the police stop me for driving under the influence of marijuana?
Many states have legalized the recreational use of marijuana and have recognized the value of legalizing medicinal marijuana use.
Despite this legalization, though, it is still a criminal offense to drive while impaired by marijuana.
Law enforcement can legally stop you for driving under the influence of weed when they have probable cause that you are driving while high.
Acts that might give rise to probable cause include you:
- weaving between lanes,
- driving on the wrong side of the road,
- running a red light,
- ignoring a stop sign, and/or
- hitting other motorists or objects.
If police stop you for driving high without probable cause, then a judge might dismiss your DUI case or reduce the charges.
5. How can a prosecutor prove I was driving while high?
Most states say that a prosecutor must successfully prove the following elements to show that you are guilty of driving under the influence of drugs:
- you drove a motor vehicle, and
- when you drove, you were under the influence of a drug (which includes marijuana).3
Prosecutors can try and show that you were “under the influence” of marijuana by using any of the following evidence:
- statements and testimony from a police officer,
- field sobriety test results,
- the results from a saliva, blood, or urine test,
- drug paraphernalia found in your car, and
- arrest reports.
For more discussion, see our page on How long does marijuana show up in a DUI blood test?
Legal References:
- See University of Pittsburgh website, “Pitt Researchers Develop Marijuana Breathalyzer Technology,” (2019).
- See, for example, CO Rev. Stat. 42-4-1301.1.
- See, for example, CALCRIM No. 2110, Judicial Council of California Criminal Jury Instructions (2021 edition); and, California Vehicle Code 23152 VC.
