DUI and Marijuana: What Is the Law and How Are THC Levels Measured in California?

What the Law Says About Marijuana and DUI in California

California Vehicle Code § 23152(f) makes it unlawful to drive under the influence of any drug, including marijuana. The law does not set a “legal limit” for marijuana the way it does for alcohol (0.08% BAC). Instead, prosecutors must prove that:

• You consumed marijuana; and
• As a result, your mental or physical abilities were so impaired that you could no longer drive with the caution of a sober person, using ordinary care, under similar circumstances.

Impairment is based on performance and observation—not just numbers on a test. It is not possible to determine marijuana impairment by chemical test results alone.

How Police Investigate Suspected Marijuana DUIs

1. Initial Observations — Odor of cannabis, red or watery eyes, slowed speech, or marijuana paraphernalia in the vehicle.
2. Field Sobriety Tests (FSTs) — Coordination tests (walk-and-turn, one-leg stand) designed for alcohol and less reliable for cannabis impairment.
3. Drug Recognition Expert (DRE) Evaluation — A 12-step protocol performed by specially trained officers; see the California Highway Patrol DRE program.
4. Chemical Testing — Typically a blood draw to measure THC and metabolites.

How THC Levels Are Measured

• No “per se” limit in California. Some states set numeric thresholds (e.g., 5 ng/mL THC in blood), but California has no statutory cutoff. See the NHTSA report on marijuana-impaired driving.
• Blood Tests. Most common method. THC is fat-soluble and can remain detectable long after acute effects subside.
• Urine Tests. Rarely used for DUI impairment because they show prior use, not present impairment.
• Emerging Tools. Breath and oral-fluid tests are being explored (see CHP’s Oral Fluid Pilot Program) but are not widely accepted in California courts.

Why Measuring THC Is Controversial

THC affects individuals differently. A casual user may be impaired at a low concentration, while a frequent user may show higher THC with little observable impairment. Because of this variability, prosecutors often rely heavily on officer testimony, driving patterns, and physical symptoms—sometimes more than numeric test results.

Common defense strategies include challenging the accuracy of blood testing and its handling, highlighting the absence of a reliable impairment threshold, and scrutinizing officer training and procedures.

Penalties for Marijuana DUI

• Fines and court assessments
• Mandatory DUI education programs
• License consequences (see the California DMV’s overview of DUI offenses)
• Possible jail time for repeat offenses or cases involving injury

Even a first offense can have lasting effects on your record and driving privileges.

Final Thoughts

While marijuana is legal in California, driving under its influence is not. The science of measuring cannabis impairment is evolving, which means there may be opportunities to challenge the evidence in a marijuana DUI case—whether by disputing test reliability, contesting observations, or identifying procedural errors.

If you’re facing a marijuana DUI, consult an experienced DUI defense attorney who understands both the law and the science.

Written by Joshua Price, published by Defending Rights Law Center®.