Drug and Cannabis DUI in California: How Impairment Is Evaluated

If you were arrested for a DUI where alcohol was not really the issue, you're probably looking at a piece of paper you believe has already settled the case. For one person it is a prescription, and it feels like proof the case should never have been brought. For another it is a blood report with a number on it, and it feels like the case is already over. Neither reading is right, and the two are wrong in opposite directions.

A prescription label may show lawful access. A blood report may show that a laboratory detected a compound or metabolite. Neither fact, by itself, answers how you were driving at the relevant time. A drug or cannabis DUI case cannot be understood from the name of a substance alone.

California's drug-only and combined-influence DUI laws are based on impairment. The prosecution must connect the claimed effect of a drug to your ability to drive, using the driving, observed behavior, timing, medical context, examination, sample collection, and toxicology evidence. Every item on that list is a place the connection can fail.

That sequence matters. When the events are taken out of order, a laboratory finding can begin to look like the conclusion rather than one part of the proof.

Start with the charge on your paperwork, not the name of the drug.

Vehicle Code section 23152(f) (opens in a new window) prohibits driving while under the influence of a drug. Section 23152(g) addresses driving under the combined influence of alcohol and a drug. A complaint may allege one or both theories, sometimes alongside an alcohol-related count.

Read the charging document before you read anything else. A drug-only theory, a combined-influence theory, an alcohol per se count, and a chemical-test refusal allegation do not present the same legal question even when they arise from the same police contact. Which one you are answering changes what matters in the file.

The word “drug” is not limited to an illegal controlled substance. Vehicle Code section 312 (opens in a new window) defines a drug by its capacity to affect the nervous system, brain, or muscles enough to appreciably impair prudent and cautious driving. That is why a medication a doctor prescribed, or something bought off a pharmacy shelf, can be the subject of a DUI charge at all. The legal inquiry can therefore involve cannabis, prescription medication, an over-the-counter product, or another substance without assuming that lawful possession or a particular label proves impairment.

The question is whether the person's driving ability was impaired.

California's 2026 criminal jury instruction for drug DUI describes a person as under the influence when a drug has affected the nervous system, brain, or muscles enough to impair, to an appreciable degree, the ability to drive with the caution characteristic of a sober person of ordinary prudence under like circumstances.

That is a functional standard. It directs attention to driving ability rather than merely to exposure or possession. The manner of driving may be considered as one part of the evidence, but the jury instruction cautions that driving conduct alone is not enough to establish that the person was under the influence.

That cuts both ways, and it's tempting to notice only the half you were hoping for. The fact that a driver used a substance does not automatically prove the charged impairment. The absence of dramatic driving or behavior does not automatically resolve the case either. The evidence must be evaluated together and in time.

California has no cannabis version of 0.08.

California law contains alcohol concentration provisions that can support a per se DUI count. It does not supply a comparable cannabis concentration that, by itself, establishes the section 23152(f) impairment element.

The California Department of Motor Vehicles explains in its cannabis-impaired-driving research materials that the severity of cannabis effects cannot be determined from the level of active compounds alone and that there is no number comparable to the 0.08 alcohol threshold. That does not make a cannabis result irrelevant, and nobody should tell you it does. It means the result must be interpreted with the collection time, the reported use history, the driving, the observations, the examination, and the limits of the analytical method.

A number in a report should therefore prompt questions, not an automatic verdict: What exactly was measured? When was the sample collected? What did the laboratory method identify? What conclusion does the witness intend to draw, and what evidence supports that step?

The gap between the driving and the sample is itself evidence.

Put every event on one chronology, in the order it actually happened:

• First, record the driving or collision.
• Next, place the stop, contact, identification, statements, and observations in order.
• Add any field or drug-recognition examination and the reported substance use, dose, or timing.
• Then add any medical assessment or treatment and the breath or blood collection.
• Finally, record the laboratory's receipt, testing, and reporting dates.

The interval between driving and the sample is part of the evidence, not background to it. So are gaps in the officer's observations, changes after a collision or medical event, and the places where the video and a later report do not match.

Use your actual records when available. Do not reconstruct medication timing from memory if pharmacy, prescribing, or treatment records can answer the question more accurately — a pharmacy record is dated, and your memory of last Tuesday is not. And do not change a medication regimen for a legal strategy; medication decisions belong with a qualified medical professional.

The DUI arrest-response guide explains how to organize the complaint, release papers, and DMV notice while the impairment record is being gathered.

Every piece of this evidence has a job, and a limit.

No single category should be asked to prove more than it can reliably show:

This division matters most when an officer's report uses a conclusion such as “under the influence” before describing the underlying observations. When you read the report, notice where that conclusion sits: placed first, it borrows authority from observations that have not been described yet. The conclusion should be traced back to what was actually seen, recorded, measured, and interpreted.

Red eyes are not a diagnosis.

Reports may describe red eyes, speech, balance, divided attention, confusion, pulse, pupil response, or performance on an examination. Reading a paragraph like that written about yourself is its own kind of awful, and it is easy to mistake it for a finding. Those observations can matter. What they are worth depends on who observed them, when, under what conditions, and whether the description is supported by video or other records.

Fatigue, anxiety, injury, environmental conditions, communication differences, and medical issues may be relevant in an individual case. They should not be invented as blanket explanations, and be careful with anyone who hands you one before looking at your records. The responsible approach is to identify a possible explanation only when the actual history or record supports investigating it.

The same principle applies to officer training and opinion testimony, and it will disappoint you in both directions. Credentials do not make every observation accurate, and an imperfect examination does not make every observation meaningless. Review the protocol used, what was omitted, the timing, the video, and the factual basis for the opinion.

Toxicology must be interpreted within the laboratory record.

A final report is not the entire laboratory file. Depending on the case, the underlying record may include sample identifiers, collection and receipt times, chain-of-custody entries, screening and confirmation methods, calibration and control material, chromatograms or instrument data, reporting limits, analyst notes, and any amended report.

The first questions are concrete, and they are yours to have answered: Is this the correct sample? Was the reported substance confirmed by the stated method? Is the result qualitative or quantitative? Were any alcohol findings also reported? What assumptions does an expert use to connect that result to the driving time?

Asking them does not accuse the laboratory of anything, and you do not have to believe a mistake was made to want them answered. They prevent a detected substance from being treated as a self-interpreting measure of driving impairment. The California DUI evidence guide explains the broader review of collection, custody, testing, video, and police-contact evidence.

Lawful use and combined influence require careful distinctions.

Vehicle Code section 23630 (opens in a new window) states that lawful entitlement to use a drug is not a defense to a DUI charge. A prescription can still be important because it may document the substance, dose, instructions, timing, history, or medical context. It does not end the impairment inquiry in either direction.

There is a matching trap on the combined-influence count. Under section 23152(g), the prosecution may rely on an alleged combined effect of alcohol and a drug. So a low alcohol reading is not the reassurance it looks like, and it therefore should not be examined in isolation when a combined-influence count is filed. At the same time, the word “combined” is not proof. The government still must connect the evidence to impaired driving ability under the charged theory.

Official Sources

• California Vehicle Code § 23152 (opens in a new window)
• California Vehicle Code § 312 (opens in a new window)
• California Vehicle Code § 23630 (opens in a new window)
• Judicial Council of California: 2026 Criminal Jury Instructions (opens in a new window)
• California DMV: Cannabis-Impaired Driving Research FAQs (opens in a new window)

Gather the paperwork, then put the opinion back in order.

Bring the complaint, reports, DMV notice, sample information, medication materials, known video details, and a careful driving-to-sample timeline to a case-specific review. The immediate task is to compare the impairment opinion with the driving, behavior, medical context, and toxicology rather than treating any one fact as a complete answer.

California DUI defense is one part of Boyadzhyan Legal Shield's criminal-defense practice. The firm offers free and confidential initial consultations, with no obligation to hire the firm. Learn more about DUI defense or request a consultation.

This article provides general information, not legal or medical advice. Drug-impaired-driving law, toxicology, medication questions, and record access require current, case-specific review by qualified professionals.