A marijuana DUI is charged under Vehicle Code 23152(f), which makes it unlawful to drive while under the influence of any drug, including cannabis. These cases are different from alcohol cases in one fundamental way: there is no legal limit for marijuana. That single fact shapes the entire defense, because the prosecution cannot rely on a number and has to prove actual impairment instead. I am Joel Brand, and here is how I take these cases apart.

There is no per se limit for cannabis

With alcohol, a 0.08 percent reading is itself a violation. Cannabis has no equivalent threshold in California. THC can remain detectable in the blood for days or even weeks after use, long after any impairing effect has worn off, especially in regular users whose bodies store and slowly release it. A blood test that shows the presence of THC therefore does not establish that a driver was impaired at the time of driving. It shows exposure, not impairment, and that gap is the central weakness in most marijuana DUI cases.

Why THC levels prove so little

Unlike alcohol, which moves through the body in a fairly predictable way, THC follows a complicated path that makes blood levels a poor proxy for impairment. A frequent user can carry measurable THC at all times, even when completely sober, while an occasional user might be impaired at a relatively low number. Levels can also drop sharply between the time of driving and the time blood is finally drawn. Scientific bodies that have studied the question have repeatedly concluded that a specific blood-THC number does not correlate reliably with impairment, which is precisely why California has never adopted a per se cannabis limit. That scientific reality is a powerful tool for the defense.

What the prosecution actually relies on

Because the blood number cannot carry the case, the prosecution leans on other evidence:

  • The arresting officer's observations of driving, demeanor, eyes, and odor.
  • Field sobriety tests, which were designed and validated for alcohol, not cannabis.
  • An evaluation by a Drug Recognition Expert, a specially trained officer who performs a structured set of checks.
  • Any statements the driver made about recent use.

Each of these is open to challenge. Driving patterns have innocent explanations, the standard field sobriety tests are a poor measure of cannabis impairment, and the Drug Recognition Expert protocol depends on the officer following every step correctly and interpreting the results fairly.

The Drug Recognition Expert evaluation

The DRE evaluation is a twelve-step process meant to identify the category of drug involved and whether the person is impaired. It looks scientific, and juries can treat it that way, but it is only as reliable as the officer who performs it. Missed steps, subjective calls, and conclusions that do not match the underlying measurements are all common, and a careful review of the DRE's own notes often shows the evaluation is weaker than the report's confident conclusion suggests. I scrutinize whether the protocol was actually followed, whether the officer was properly certified, and whether the physical findings have innocent explanations.

Innocent explanations the prosecution ignores

Many of the cues an officer attributes to cannabis have ordinary causes. Red or watery eyes can come from fatigue, allergies, smoke, or contact lenses. Nervousness at a traffic stop is universal and is not evidence of being high. The odor of cannabis, even if present, establishes exposure or possession at some point, not impairment while driving. A driver who recently used cannabis legally at home hours earlier may smell of it and carry THC in their blood while being completely unimpaired behind the wheel. I make sure these alternative explanations are developed, because the prosecution's narrative depends on treating every neutral fact as proof of impairment.

How these cases get defended

The defense starts with the stop and the arrest, then focuses on the disconnect between the presence of THC and proof of impairment. The timing of use, the driver's tolerance, the limits of the field testing, and the soundness of any DRE evaluation all come into play. Prescription and lawful adult use are relevant context, because using cannabis legally is not a crime; driving while actually impaired is what the statute requires. Where the impairment evidence is thin, and in marijuana cases it very often is, these cases can be highly defensible. See my top DUI defenses.

Edibles and the timing trap

Edibles add another layer of complication that often helps the defense. Because they are absorbed through the digestive system, edibles take effect slowly and their peak comes long after consumption, so a driver may have eaten one hours earlier and be well past any peak effect by the time they are stopped, even while THC is present in the blood. The mismatch between when cannabis was consumed, when it actually affected the person, and when blood was finally drawn makes any inference of impairment at the time of driving especially shaky. I look closely at the form of cannabis involved and the timeline, because that sequence frequently undercuts the prosecution's impairment theory.

Combined alcohol and cannabis cases

Many marijuana cases are not pure drug cases at all but combined ones, where a driver had some alcohol and some cannabis and the prosecution argues the two together caused impairment. These combined theories are often a sign that neither substance was strong enough to carry a case on its own, so the state hopes the combination adds up to a conviction. The same weaknesses apply to each half: the alcohol level may be too low to mean much, and the THC shows only exposure rather than effect. Pulling the two apart and holding the prosecution to proof of genuine combined impairment frequently exposes how thin the theory really is. See combined alcohol and drug DUI.

Where to start

A marijuana DUI rises and falls on proof of impairment, not on a number, and that gives a well-prepared defense real room to work. The absence of a per se limit, the poor correlation between THC levels and impairment, and the soft science behind the field and DRE evidence all combine to make these cases far more contestable than the prosecution would like a jury to believe. Use the free case analysis on this page, or call me directly at (888) 271-6644. I answer my own phone, 24/7. It may also help to read about how drugs affect a DUI defense and field sobriety tests and your right to decline them.

Related: drug possession (HS 11350).