What levels of prescription drugs can a person be charged with driving under the influence of prescription drugs in Calif?

Any level that influences or affects driving. Different drugs have different effects and to a certain degree, affect some people more than others. Most DUI laws are drug-independent and are concerned with whether the drug you are taking has had any deleterious effects to your ability to correctly drive. Regardless of the actual level of drug circulating in your blood stream, if you fail a field sobriety test or can be otherwise shown to be operating at a deficit, then you can be charged. If the drug you are taking carries a label cautioning against driving or operating machinery while taking it, then it is likely that even at a therapeutic level, you are a risk to yourself and other drivers on the road. People who feel their prescription medicine may have blunted their mental acuity or reflexes, should not drive until they have completed the course of medication and given sufficient time for the medicine to clear their systems.