Marijuana is legal for adults in California, but driving under its influence is not, and a cannabis DUI works very differently from an alcohol case. I am Joel Brand, and I defend DUI cases across the state. The science is far murkier, there is no clean legal number, and that cuts in surprising directions. If you were arrested for driving after using cannabis, here is what makes these cases distinct, and often quite defensible.
Legal to use, illegal to drive impaired
Legalization changed possession and use, but it did not change the rule that you cannot drive while impaired by any substance. A marijuana DUI is charged under the rule I explain in marijuana DUI under Vehicle Code 23152(f), which prohibits driving under the influence of a drug. Having a medical recommendation or being a legal adult user is not a defense to driving impaired, but it also does not make you automatically guilty just because cannabis was in your system.
There is no per se limit for THC
This is the single biggest difference from an alcohol case. For alcohol, the law sets a clean 0.08 per se number, and being at or above it is the offense by itself. For marijuana, California has not set any equivalent legal threshold. There is no number that automatically makes you guilty. That means a cannabis DUI is almost always about impairment, and the prosecution has to prove you were actually under the influence, not just that THC was present.
Why THC blood levels do not prove impairment
Unlike alcohol, THC does not correlate neatly with impairment, and its levels in the blood follow a very different pattern. THC can spike and then drop quickly even while effects continue, and for regular users it can linger in the body long after any impairment has passed. So a blood test showing THC tells you that cannabis was used at some point, but it does not reliably tell you whether you were impaired at the moment you were driving. This gap is central to defending these cases.
The active versus inactive problem
Blood tests can detect both active THC and inactive metabolites, and the difference matters enormously. Inactive metabolites can remain detectable for days or even weeks after use, long after any effect is gone. A result that does not carefully distinguish active compound from lingering metabolites can make a sober driver look like an impaired one. Scrutinizing exactly what the lab measured, and what it actually means, is a key part of the defense.
The drug recognition evaluation
Because there is no number to rely on, marijuana cases lean heavily on a drug recognition evaluation, a structured set of observations and tests performed by an officer with special training. These evaluations sound scientific, but they are subjective and depend on the officer's interpretation of physical signs. Each step of that evaluation can be questioned, from how it was conducted to whether the conclusions actually follow from the observations. The officer's opinion is evidence, not proof.
Field sobriety tests were not built for cannabis
The standardized field sobriety tests were developed and validated for alcohol impairment, not for marijuana. Applying them to a cannabis case imports all the usual problems with these tests, subjectivity, condition sensitivity, anxiety, and adds the question of whether they even measure what the officer claims for this substance. The mismatch between alcohol-based tools and a cannabis allegation is itself a line of defense.
Innocent explanations are everywhere
Many of the signs officers attribute to cannabis impairment, red eyes, dry mouth, nervousness, an odor, have perfectly innocent explanations. The smell of cannabis in a car does not establish that the driver was impaired while driving, and physical signs can come from allergies, fatigue, the stress of a stop, or simply having used cannabis legally hours earlier with no lingering effect. Separating actual impairment from these ambiguous signs is where these cases turn.
Combining cannabis with anything else
Cases get more complicated, and sometimes more serious, when cannabis is combined with alcohol or other substances. The interaction can change how the case is charged and defended, and prescription medication adds its own layer, which I address in how prescription drugs affect your DUI defense. Understanding exactly what is alleged to have been in your system, and in what combination, shapes the whole strategy.
The same procedural defenses still apply
On top of the marijuana-specific issues, every ordinary DUI defense remains available. The stop still has to be lawful, the blood draw still has to follow proper procedure, the chain of custody still matters, and your rights still apply. I survey the full range in my guide to the top DUI defenses. A cannabis case often has both the standard defenses and the extra uncertainty of the science working in its favor.
Why these cases are often more defensible
Put it all together and a marijuana DUI frequently has more reasonable doubt baked in than an alcohol case. No per se number, blood levels that do not track impairment, subjective evaluations, and tests built for a different substance all give the defense real room to work. That does not mean these cases are easy, but it does mean they should rarely be treated as hopeless, and a quick guilty plea often gives up a genuinely contestable case.
The bottom line
A California marijuana DUI is about impairment, not a number, and the science connecting THC to impairment is weak, which often works in your favor. Between the absence of a per se limit and the subjectivity of the evidence, these cases are frequently very defensible. If you were arrested for a cannabis DUI, get a free written case analysis below, or call me directly at (888) 271-6644. I answer my own phone, 24/7. You can also read more from the DUI blog.