In California, drivers are considered to be under the influence of drugs if they have a blood concentration of more than 0.01% THC, the active ingredient in marijuana. Drivers can be arrested and charged with DUI if they are found to be impaired by any drug, including prescription medications.
To test for driving while high, police officers will typically use a roadside sobriety test. This test is designed to assess a driver's ability to safely operate a vehicle. Officers may also use a field sobriety test, which is a series of physical and mental exercises that must be performed in order to determine impairment.
If an officer has reasonable suspicion that a driver is under the influence of drugs, they may request a chemical test of the driver's blood or urine. These tests can determine the presence and concentration of drugs in the body. Drivers who refuse to submit to chemical testing can be subject to penalties, such as license suspension.