State DUI laws apply to driving while under the influence of alcohol or illegal drugs. But you can also get a DUI for operating a vehicle while under the influence of legal drugs, including prescription and over-the-counter medications.
DUI Laws that Apply to Prescription and Legal Drug Use
All states have two types of DUI laws: "per se" and impairment.
Per Se DUI Laws
Per se laws make it illegal to drive with a certain concentration of a controlled substance in your system or with a blood alcohol concentration (BAC) of .08% or more (.05% or more in Utah). For most legal drugs and medications, per se DUI laws aren't applicable. But, in a few states—like Washington and Nevada—where marijuana use is legal, there are per se laws that make it illegal to drive with a certain concentration of THC (the primary psychoactive ingredient in marijuana) in your system.
Impairment DUI Laws
Generally, a DUI charge related to lawful drug or medication use will be based on the driver's actual impairment. In other words, the prosecution needs to prove the driver was actually intoxicated from the medication or drug ingested. In most states, slight impairment isn't enough—the prosecution must show the driver was to the point of being less able to operate a vehicle safely. (However, in states like New York and Colorado, it's also illegal to drive while even slightly impaired.)
To prove an impairment DUI, prosecutions might present the testimony of the arresting officer related to the motorist's physical appearance, manner of driving, and field sobriety test (FST) performance.
Fighting a Prescription Drug DUI Charge
Defenses to impairment DUI charges normally focus on providing innocuous explanations for observations the arresting officer attributed to intoxication. For example, a driver might provide evidence of physical disabilities to explain poor field sobriety test performance. Or, a motorist could argue he or she appeared intoxicated because of nervousness or mental distress.
But with prescription drug DUIs, a number of states provide an additional defense for drivers who are charged with driving under the influence. In these states—which include Arizona, Louisiana, Ohio, Iowa, and North Dakota—a driver can beat a DUI charge by proving he or she was impaired due to a prescription medication taken as prescribed by a doctor.
Even in states that don't have a prescription medication defense, proof that the driver's intoxication was attributable to a drug prescribed by a physician could help in the plea bargaining process or be considered a mitigating circumstance justifying lenient penalties.
