In a marijuana DUI case, police attempt to prove impairment by observing driving behavior, conducting field sobriety tests, evaluating physical signs of intoxication, and, in many cases, relying on a specially trained officer known as a Drug Recognition Expert (DRE).
Because there is no universally accepted legal limit for THC like there is for alcohol, proving a marijuana DUI often involves subjective assessments backed by toxicology results and officer testimony.
Understanding how these arrests are handled can help individuals prepare for the legal and procedural steps that follow. Speaking with a Los Angeles marijuana DUI lawyer can provide insight into how marijuana-related DUI charges are evaluated and challenged in California.
Marijuana vs. Alcohol DUIs: What’s Different?
Unlike alcohol DUIs, which rely on a measurable blood alcohol concentration (BAC) level, marijuana DUIs are based on observed impairment rather than a specific chemical threshold. While a blood or urine test may show the presence of THC, that doesn’t necessarily prove a person was impaired at the time of driving.
THC can remain in a person’s system for hours, days, or even weeks after use, depending on how frequently they consume cannabis. This makes proving actual impairment more complicated for police and prosecutors—and more open to legal challenge.
What Officers Look for During a Stop
When police suspect a driver is under the influence of marijuana, the first step is often visual observation and questioning. Some of the signs an officer may cite include:
- Erratic or slow driving: Drifting within or between lanes, delayed responses.
- Odor of marijuana: Strong smell of cannabis coming from the vehicle or person.
- Bloodshot eyes: Often associated with recent marijuana use.
- Slurred speech or lethargy: Slow or confused responses.
- Unusual behavior: Nervousness, paranoia, or poor motor coordination.
These observations alone are not enough to secure a conviction, but they do form the foundation for additional testing and probable cause for arrest.
Field Sobriety Tests and their Limitations
If marijuana use is suspected, police often administer field sobriety tests (FSTs). These standardized exercises include:
- Horizontal Gaze Nystagmus (HGN): Tests eye movement, though it’s more relevant for alcohol.
- Walk–and–turn test: Evaluates balance, coordination, and the ability to follow instructions.
- One–leg stand: Assesses balance and focus.
Although FSTs are a routine part of DUI stops, they aren’t specifically designed for detecting marijuana impairment and can be influenced by factors like anxiety, fatigue, or medical conditions.
Role of a Drug Recognition Expert (DRE)
In many marijuana DUI cases, a Drug Recognition Expert is called in to perform a more detailed assessment. A DRE is trained to identify drug-specific impairment through a 12-step evaluation, which includes:
- Measuring pupil size and checking for involuntary eye movement.
- Monitoring blood pressure and pulse.
- Evaluating muscle tone and behavior.
- Requesting a toxicology test.
While a DRE’s opinion can carry weight in court, it’s still based on interpretation rather than a scientific measurement of current impairment. These evaluations can be challenged if procedures weren’t followed correctly or if the DRE lacks proper training.
Blood Tests and THC Levels
A blood test may be used to confirm the presence of THC or its metabolites. However, California law does not define a specific THC limit that constitutes legal impairment. This makes marijuana DUI cases more subjective and complex than alcohol DUIs.
Even with a positive test result, the defense may argue that the presence of THC does not correlate with impairment at the time of driving. For frequent cannabis users, residual THC levels may appear in the blood long after any intoxicating effects have worn off.
Building a Marijuana DUI Case
To prove impairment, the prosecution typically relies on a combination of the following:
- Officer testimony about observed behavior.
- Field sobriety test results.
- DRE evaluations (if conducted).
- Blood test results showing THC.
- Video footage from the traffic stop or body cam.
- Statements made by the driver.
Each piece of evidence plays a role, but none is usually conclusive on its own. That’s why marijuana DUI cases often focus on the totality of the circumstances and leave room for interpretation.
Challenging Marijuana DUI Charges
Because there’s no definitive THC limit for impairment, defense strategies often focus on questioning the reliability of the arrest process. Common defenses include:
- Arguing that observed behavior was unrelated to marijuana use.
- Disputing the accuracy or administration of field sobriety tests.
- Challenging the qualifications or findings of the Drug Recognition Expert.
- Providing evidence of non-impairing THC presence due to medical use or chronic use.
- Highlighting inconsistencies in police reports or video footage.
Understanding these potential defenses can help individuals facing charges assess their options early in the process.
Know What to Expect
Marijuana DUI cases rely heavily on officer judgment, subjective testing, and medical interpretation. These variables make the charges more flexible—but also more open to dispute. Understanding how the police prove impairment in a marijuana DUI can help individuals prepare for what lies ahead.
Los Angeles DUI Lawyer can connect you with an experienced attorney who understands the challenges of marijuana-related DUI cases in California. With the right support, it may be possible to challenge the evidence or reduce the impact of the charge.
