What is a drug DUI and how do they prove it?

The elements are the same as an alcohol DUI. What changes entirely is the proof.

There's no per se threshold for most substances, so the State generally has to prove actual impairment of normal faculties rather than a number. Its evidence typically comes in three layers: the arresting officer's observations, an evaluation by a drug recognition expert, and a urine or blood analysis.

Each layer has real limitations. Officer observations of non-alcohol impairment are less familiar territory than alcohol and more open to alternative explanations. The drug recognition evaluation is a structured protocol whose reliability depends on whether it was performed correctly. And chemical testing generally establishes presence rather than current impairment — some substances remain detectable long after any effect has ended.

That final gap is where these cases are usually fought. A positive result and impairment at the time of driving are different propositions, and the State has to prove the second.