The same statute that makes drunk driving a crime in California also covers "drugged driving." This law makes it unlawful to drive while:
Under the influence. For purposes of California's drugged driving law, "under the influence" means the substances ingested have "so far affected the nervous system, the brain, or muscles as to impair to an appreciable degree" the person's ability to operate a vehicle safely.
Addiction. Drugged driving charges based on addiction aren't common. But as used in the statute, "addicted to the use of any drug" refers to the compulsive dependence on a drug. Indications of such a dependence might include withdrawal symptoms, increased tolerance, and an uncontrollable need to continue using drugs. Generally, the statute doesn't prohibit individuals participating in drug recovery programs from lawfully driving.
Basically, California law defines the term "drug" as any substance or combination of substances, other than alcohol, that can impair a person's ability to drive safely to an "appreciable degree." Under this broad definition, even herbal supplements like kava can qualify as a "drug." Over-the-counter and prescription drugs (like painkillers and medical marijuana) may also fit the definition. So it's not an acceptable defense to a drugged driving charge to claim that the driver was legally entitled to use the substance.
A driver arrested for drugged driving in California will be charged with driving under the influence and typically be subject to the same penalties that apply to an alcohol DUI. And a conviction for drugged driving will be considered a prior offense for purposes of calculating punishments. So generally, the costs and other consequences of a drug-DUI conviction depend on whether it's a first, second, or third offense.
California's "implied consent" law requires all drivers lawfully arrested for driving under the influence of drugs or alcohol to submit to blood or urine testing when requested to do so by an officer. Refusal to take a test results in license suspension and can be admitted into evidence against the driver at a DUI trial.