While people often associate DUI arrests with alcohol, drivers can be arrested for driving under the influence of drugs, too. The drugs do not even need to be controlled substances. In California, it is illegal to drive under the influence of a number of legal drugs--such as cough medicine, Ambien and medical marijuana. Drug-related DUI cases are fairly complicated because there generally is no legal threshold of drug use. For example, it is illegal to drive with a blood alcohol content of .08 percent or more, but there is no concrete limit when it comes to marijuana use.
In Washington state, where recreational marijuana use was legalized last year, it is illegal to drive with more than 5 nanograms of THC--the active ingredient in marijuana--per milliliter of blood. Here in California, there is no established limit; instead, the law generally states that a person can be convicted of driving under the influence of marijuana if he or she was not able to drive with the same care and caution as a sober person.