A man in California was recently charged with a DUI, after a blood test came up positive for only caffeine.  This became a national news story, and was featured on CNN, and although his case was later dismissed by the local district attorney, they did not back down on their assertion that a person could be under the influence with such a blood test result.  They stated that the officer believed he was under the influence of a drug, and the blood test only tests for some substances.  

Here at Newton & Hall, we have seen many people charged with DUI or similar offenses when they are alleged to have been under the influence of a drug that does not fall into the normal categories of alcohol, marijuana, or other illegal substances.  Many times our clients are taking a prescription medication for pain, anxiety, depression, or other medical condition, and they don't know that this can result in a DUI.  Officers on scene, especially after car accidents, are trained to look for evidence of medications and inquire of drivers regarding what medications they are taking. 

The statute that governs DUI cases is RCW 46.61.502, which states that :

(1) A person is guilty of driving while under the influence of intoxicating liquor, marijuana, or any drug if the person drives a vehicle within this state:

(a) And the person has, within two hours after driving, an alcohol concentration of 0.08 or higher as shown by analysis of the person's breath or blood made under RCW 46.61.506; or
(b) The person has, within two hours after driving, a THC concentration of 5.00 or higher as shown by analysis of the person's blood made under RCW 46.61.506; or
(c) While the person is under the influence of or affected by intoxicating liquor, marijuana, or any drug; or
(d) While the person is under the combined influence of or affected by intoxicating liquor, marijuana, and any drug.

(2) The fact that a person charged with a violation of this section is or has been entitled to use a drug under the laws of this state shall not constitute a defense against a charge of violating this section.

As you can see, being under the influence or affected by any drug can get you a DUI charge.  This is the case even if you are allowed to use the drug.  We often have clients say that are only "slightly" or "barely" affected by drugs or alcohol when questioned by an officer.  This is akin to a confession, since they are admitting that they are affected by the drug or alcohol.  An enthusiastic prosecutor could certainly argue that caffeine is a drug.  

For this reason, we advise all of our clients to not answer questions about what they may or may not have taken, and immediately ask to speak with their attorney.  An experience washington DUI defense lawyer can be of great assistance in these cases, but your best chance to beat a DUI is to never get one to begin with!