While the majority of DUI offenses stem from the use of alcohol or illegal drugs, it's becoming common to get a DUI in Seattle for driving under the influence of anything that leaves you impaired. This means that a Seattle driver runs the risk of a DUI by driving while under the influence of perfectly legal prescription or over-the-counter medications.

Possessing and ingesting over-the-counter and prescription medications is legal, but so is alcohol. So, like alcohol, if an officer believes that prescriptions have impaired driving, it can still result in a DUI. The penalties for driving under the influence of over-the-counter or prescription medication are the same as driving under the influence of alcohol or illegal drugs.

Even though both may have impairing effects, medication does differ from alcohol in a DUI case. For alcohol, detection of it's the BAC can be done by breath or blood test and 0.08 is a known legal limit throughout the country. For medicated driving, there is no agreed-upon limit for which impairment has been scientifically calculated. Furthermore, determining current drug levels can be difficult, since some drugs linger in the body for a period of days or weeks after initial ingestion.

Some states have made it illegal to drive with any detectable level of prohibited drugs in the blood. But setting any kind of limit for prescription medications is far more complicated, partly because the complex chemistry of drugs makes their effects more difficult to predict than that of alcohol. Determining whether a driver took drugs soon before getting on the road can be tricky, since some linger in the body for days or weeks.

There is no reliable data on how many drivers are impaired by prescription drugs, but law enforcement agencies are putting police officers through special training to spot signs of drug impairment and clamoring for better technology to detect it. Washington is among those states that now has trained drug recognition experts (“DRE”) trained to spot signs of impairment in drivers.

The police may struggle, however, with the challenge of prosecuting someone who was taking valid prescriptions. In an interview with the New York Times, Mark Neil, senior attorney at the National Traffic Law Center, stated: “How do we balance between people who legitimately need their prescriptions and protecting the public? It becomes a very delicate balance.”

Police are arguably casting too wide a net and unfairly punishing people who are taking prescriptions as directed. With Washington's vague definition of “impairment” including “driving while under the influence of drugs,” police have great discretion in who they decide to stop and arrest for an alleged DUI charge.

References:
http://www.nytimes.com/2010/07/25/us/25drugged.html?_r=1