You don't have to drink alcohol to be charged with a DUI in California.  Any drug, legally prescribed or over the counter, as wel as illegal, can impair one's driving abilities. The difference with a driving under the influence of drugs is how to prove it in court. 

The easiest evidence a district attorney has is a blood test.  Lacking that, the state is forced to show a person is under the influence by looking at the person's driving,  Erratic driving patterns.  Tell tale signs of being under the influence of a drug.  Field Sobriety tests.

A typical police officer will not be able to testify as to whether a driver is under the influence.  Typically an officer with training will be called in.  These officers are called Drug Recognition Evaluators, and must follow specific procedures to evaluate the driver. 

In California, you will need a qualified defense attorney to make sure procedures were followed, and be prepared to question the final results of the officer.