Legal drug use and prescription medications can lead to a DUI/DWI conviction. Here's why.
All states have two types of standard DUI charge: “per se” and “impairment.” A per se DUI charge is based on the amount of drugs or alcohol in the driver’s system. So, a driver can be convicted of a per se DUI for having a blood alcohol concentration (BAC) of .08% or more, regardless of whether the driver was drunk. And some states have per se drug DUI laws that prohibit driving with a certain blood concentration of illegal drugs.
An impairment DUI charge, on the other hand, is based on the actual effects that the drugs or alcohol had on the driver. States have different ways of defining impairment. But in many states, a driver is considered “under the influence” if the substances ingested had a “substantial effect” on the driver. In other words, the drugs or alcohol had a significant impact on the driver’s ability to operate a motor vehicle.
The impairment DUI laws of most states apply to all intoxicating substances. So, it generally doesn’t matter whether the driver’s impairment was due to a legal or an illegal substance or whether the driver had a prescription from a doctor.
(Also, read about how marijuana legalization has affected state DUI laws.)
That being said, some states do have a limited DUI defense for prescription medications. To establish the defense, the driver generally needs to prove that he or she took the medication in the amount and manner prescribed by a physician. In practice, the defense is rarely applicable because medications that have the potential to impair a person’s ability to drive usually say so on the label. A warning label would likely prevent a defendant from being able to use the defense in court.