Drunk and drugged driving charges are based on chemical test results rather than proof of actual impairment.
Updated 12/04/2025
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A "per se" DUI (driving under the influence) charge is based on a certain amount of drugs or alcohol in a driver's blood. All states have DUI laws that prohibit driving with a blood alcohol concentration (BAC) of .08% or more (.05% or more in Utah). And several states—including Ohio, Washington, and Georgia—also have laws making it illegal to operate a vehicle with a certain concentration of drugs in your system.
Proof of Impairment Not Required
Though it's also a crime to drive while impaired by drugs or alcohol, per se DUI laws don't require proof that the driver was actually affected by the drugs or alcohol ingested. To get a per se DUI conviction, the prosecution just needs to show the motorists had a prohibited concentration of drugs or alcohol in their bodies while driving. And, with drug DUI laws in some states—like Oklahoma, Pennsylvania, and Illinois—any amount of a controlled substance in your system can lead to a conviction.
Implied Consent Laws
Implied consent laws generally require all drivers lawfully arrested for driving under the influence to take a BAC test (typically, a blood or breath test) if asked to do so by law enforcement. Under implied consent laws, drivers who refuse a chemical test after a lawful DUI arrest will have their license suspended. In some states, refusal also is a crime that can lead to jail time, but the United States Supreme Court has prohibited states from imposing criminal penalties for refusing a blood test (unlike breath and urine tests). (Birchfield v. North Dakota, 579 U.S. 438 (2016).)
So, in most DUI cases, the prosecution will have test results showing the amount of drugs or alcohol in the driver's system. Test results are crucial evidence for proving a per se DUI charge.
Proving and Defending Against a Per Se DUI Charge
Sometimes, BAC test results make it easy for the prosecution to prove a DUI charge. But in other cases, defense attorneys can poke holes in the prosecution's evidence by finding problems with the testing equipment or procedures. The "rising-blood-alcohol defense," for example, is often used by defense attorneys to challenge BAC evidence.
Getting Help From a DUI Attorney
Every case is different. So, the best way to find out where you stand is to talk to an experienced DUI attorney. An attorney can look at the case and determine whether you have any defenses.
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