Vehicle Code section 23152(f) makes it a crime to drive under the influence of any drug. Section 23152(g) covers the combined influence of alcohol and a drug. The word drug is broad: it reaches cannabis, prescription medication taken exactly as directed, and over-the-counter sleep aids and antihistamines, not only controlled substances.
What these cases lack is the thing that makes an alcohol DUI easy to prosecute. There is no per se limit for drugs in California. No statute says a particular concentration of THC or any other substance in your blood establishes impairment. The prosecution has to prove that the drug actually impaired your ability to drive with the caution of a sober person, and it has to do that with observation and expert opinion rather than a number. Marc Kohnen has been defending these cases in San Diego since 2008.
A Prescription Is Not a Defense — and Not the End of the Analysis
Having a valid prescription does not exempt you from section 23152(f). If a medication impaired your driving, the charge applies whether a doctor prescribed it or not, and the same is true of a physician's recommendation for cannabis or lawful adult use under state law. Legal to possess and legal to consume is not legal to drive impaired.
That said, lawful use does change the evidentiary picture in ways that help. A patient on a stable, long-term dose develops tolerance, and the presence of a therapeutic level of a prescribed medication says very little about impairment on the day in question. Prescription records, dosing history, and the treating physician's observations become defense evidence rather than prosecution evidence.
Why Cannabis Cases Are the Weakest
Cannabis DUI prosecutions have a scientific problem at their center. THC does not behave like alcohol in the body. Blood alcohol concentration correlates reasonably well with impairment and declines on a predictable curve. THC does not: blood levels peak and fall sharply while subjective and functional effects lag behind, so a blood level drawn an hour or two after driving tells you very little about the moment behind the wheel.
The metabolite problem compounds it. Laboratories frequently report carboxy-THC, an inactive metabolite that can remain detectable for days or weeks in a regular user and does not cause impairment at all. A positive result reported without distinguishing active THC from inactive metabolites establishes past exposure, not impairment during driving. Tolerance among regular and medical users widens the gap further. These are the points on which cannabis DUI cases are defensible, and they require someone willing to litigate the toxicology rather than accept the report.
The Drug Recognition Expert Evaluation
Because there is no number, the prosecution usually leans on a Drug Recognition Expert — an officer trained in a standardized multi-step evaluation involving vital signs, pupil measurements, divided-attention exercises, and an opinion identifying a category of drug.
The title carries more weight with a jury than the qualification deserves, and that gap is a defense opportunity. A DRE is a police officer who completed a training course, not a physician or a toxicologist. The protocol has specific requirements for how each step is performed and documented, and departures from it are common and discoverable. Many of the physical signs the evaluation relies on — elevated pulse, dilated pupils, poor balance, red eyes — are equally consistent with fatigue, anxiety about being arrested, illness, allergies, a head injury from a collision, or a medical condition such as diabetes. The question worth asking of every DRE opinion is what evidence ruled those alternatives out.
- Was the full evaluation actually completed and contemporaneously documented, or reconstructed afterward
- Did the officer complete the steps in the order and manner the protocol requires
- Were innocent medical explanations for the observed signs considered and excluded
- Does the laboratory report distinguish active compounds from inactive metabolites
- How much time passed between driving and the blood draw
- Does the toxicology actually corroborate the category of drug the DRE identified
The Blood Test and Title 17
Drug DUI cases almost always rest on a blood draw, which brings its own set of requirements. Title 17 of the California Code of Regulations governs how blood is collected, preserved, sealed, stored, and analyzed. Compliance is not presumed — it is provable or it is not, and the records exist.
There is also a constitutional layer. A warrantless blood draw ordinarily requires a warrant or a recognized exception, and the natural dissipation of a substance in the bloodstream does not by itself supply the emergency that would excuse getting one. Where blood was taken without a warrant and without valid consent, suppression is on the table. Independent retesting of the sample is often worth pursuing as well, and the sample does not last forever.
The Charge Can Come With Others
A drug DUI arrest frequently arrives with additional counts — possession of a controlled substance, possession of paraphernalia, or in some cases child endangerment where a child was in the vehicle. Those charges have their own defenses, and some carry diversion or treatment options that the DUI count itself does not.
That combination can work in your favor. Vehicle Code section 23640 bars diversion for the DUI count, but a companion possession charge may be eligible for pretrial diversion under Penal Code section 1000, and veterans and active-duty service members with a service-related condition may qualify for military diversion under Penal Code section 1001.80, which does reach misdemeanor DUI. Resolving the whole package requires looking at every count together rather than one at a time.
Talk to Marc About Your Case
Drug DUI cases are won on the science and on the paperwork — the toxicology report, the DRE's documentation, the blood collection records, and the video of the roadside evaluation. All of it has to be requested, and some of it does not survive long if nobody asks.
The DMV also imposes its own ten-day deadline to request an administrative hearing after the arrest. If you were arrested for driving under the influence of drugs or medication anywhere in San Diego County, call (619) 398-2500 for a free, confidential consultation. Marc is available 24 hours.
More on DUI & DWI
- DMV Hearings — Ten days to demand a hearing under VC §13558 — or the suspension takes effect without one.
- First Offense DUI — VC §23536 penalties, probation terms, and where a first-offense case is actually won.
- Felony DUI — Injury, a fourth offense in ten years, or a prior felony DUI — state prison exposure under VC §23153 and §23550.
- Chemical Test Refusal — VC §23612 implied consent — a one-year suspension that outlives a dismissal, and how refusals are challenged.
- Underage DUI — Zero tolerance at 0.01% under VC §23136 — a one-year suspension with no conviction required.
Back to the DUI & DWI overview.
Legally reviewed by Marc S. Kohnen, Attorney at Law — State Bar of California #255303, defending San Diego since 2008.
Free, confidential case evaluation — available 24 hours at (619) 398-2500.