Yes, recreational marijuana is legal in Massachusetts since 2016. And yes, you can absolutely be arrested, charged, and convicted of OUI for driving under the influence of marijuana. This is one of the most common misconceptions we see in criminal defense—people assume that because possession is legal, driving after using marijuana is somehow less serious than driving after drinking. It isn’t.
An OUI-marijuana charge carries the same penalties as an OUI-alcohol charge, including license suspension, probation, and jail time. The difference is how these cases are investigated, prosecuted, and defended.
How Police Detect Marijuana Impairment
There is no breathalyzer for marijuana. Unlike alcohol, where an officer can administer a roadside breath test and get a number, marijuana impairment is detected almost entirely through observation—and that distinction shapes every aspect of these cases.
An OUI-marijuana investigation typically starts the same way any traffic stop does. An officer pulls you over for a traffic violation or erratic driving and approaches the vehicle. From there, the officer is looking for indicators of impairment: the smell of marijuana, bloodshot or glassy eyes, slow or confused speech, fumbling with documents, and general demeanor. If the officer suspects impairment, you’ll likely be asked to perform field sobriety tests.
Here’s where it gets complicated. The standard field sobriety tests—the walk-and-turn, the one-leg stand, and the horizontal gaze nystagmus test—were designed and validated for alcohol impairment.
The Massachusetts Supreme Judicial Court recognized this directly in Commonwealth v. Gerhardt (2017), holding that officers can testify about a driver’s performance on field sobriety tests in a marijuana case, but cannot offer an opinion that the driver “passed” or “failed” those tests. The court acknowledged that the scientific literature does not support using these tests to reliably measure marijuana impairment the way they measure alcohol impairment.
That ruling matters. It limits what the Commonwealth can argue at trial based on field sobriety evidence alone.
No Per Se Limit—and Why That Changes Everything
With alcohol, Massachusetts law draws a clear line. If your blood alcohol content is .08 or above, you are legally impaired. Period. The prosecution doesn’t need to prove anything else about your driving or behavior. That’s what “per se” means—the number alone is enough.
Marijuana has no equivalent. There is no nanogram-per-milliliter threshold in Massachusetts law that makes you automatically guilty of OUI-marijuana. THC metabolizes differently than alcohol, and blood or urine levels of THC don’t correlate reliably with real-time impairment. A regular marijuana user can have THC in their system for days or even weeks after last using, long after any impairing effects have worn off. A positive blood test proves prior use. It does not prove impairment at the time of driving.
This is the single biggest difference between an OUI-alcohol case and an OUI-marijuana case. Without a per se limit, the prosecution must prove impairment through other evidence—officer observations, driving behavior, field sobriety test performance, and sometimes expert testimony. The entire case becomes more subjective, and subjectivity cuts both ways.
Drug Recognition Experts
In some marijuana OUI cases, police will call in a Drug Recognition Expert—a DRE. These are officers who have completed a standardized training program through the International Association of Chiefs of Police that teaches a twelve-step evaluation protocol for identifying drug impairment. The evaluation includes checking vital signs, examining pupil size and reaction, assessing muscle tone, and conducting a series of divided-attention tests.
DRE testimony can be persuasive to a jury, but it has real limitations. The twelve-step protocol was developed primarily through law enforcement training, not independent scientific research, and its reliability has been challenged in courts around the country. Defense attorneys can and should scrutinize a DRE’s training, the specific steps followed in the evaluation, whether the protocol was performed correctly, and whether the officer’s conclusions are actually supported by the observations documented. A DRE opinion is not a blood test result. It is one officer’s assessment, and it is subject to cross-examination like any other opinion testimony.
The Challenges of Prosecution
Marijuana OUI cases are genuinely harder for the Commonwealth to prove than alcohol cases. There’s no breath test number to put in front of a jury. Field sobriety tests can’t be presented the same way under Gerhardt. Blood test results don’t prove impairment at the time of driving. That leaves prosecutors building their case primarily on the arresting officer’s observations and, in some cases, DRE testimony.
That evidence can be enough to secure a conviction—don’t underestimate it. But it creates more room for reasonable doubt than a .14 BAC result does. Jurors understand numbers. Asking twelve people to convict based on an officer’s description of bloodshot eyes and the smell of marijuana in the car is a different proposition entirely.
The Challenges of Defense
If the prosecution’s case is harder to build, the defense side has its own set of challenges. The most significant one is the assumption many jurors carry into the courtroom: if you smelled like marijuana and performed poorly on field sobriety tests, you were probably impaired. Overcoming that gut reaction requires education—explaining to the jury that THC affects the body differently than alcohol, that field sobriety tests aren’t validated for marijuana, and that the odor of marijuana tells you nothing about when it was used or whether someone is currently impaired.
The defense also has to confront the reality that Massachusetts has no clear legal standard for what marijuana impairment looks like. That ambiguity helps in some ways—there’s no number to point to—but it also means the jury is largely deciding based on impressions, and impressions can be unpredictable.
What to Do If You're Charged
An OUI-marijuana charge is a criminal offense with serious consequences. It goes on your criminal record. It triggers a license suspension. And because these cases depend so heavily on subjective evidence, the quality of your defense matters as much as the facts. Having an attorney who understands the science behind THC, the limits of field sobriety testing, and the current state of Massachusetts case law on marijuana impairment can make the difference between a conviction and a favorable outcome.
If you’ve been charged with OUI-marijuana in Eastern Massachusetts, call a Massachusetts criminal defense attorney today. Not tomorrow. Before your next conversation with anyone about this case. Contact the Law Office of Matthew Peterson today for a consultation.
