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.

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. 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.

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 conclusion suggests.

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, these cases can be very defensible.

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. You can get a free written case analysis below or call me directly. It may also help to read about how drugs affect a DUI defense and field sobriety tests and your right to decline them.