A charge of Driving Under the Influence (DUI) may arise after the consumption of alcohol or drugs, both legal and illegal. Alcohol impairment is established with a BAC reading of .08 or higher.
It can also be proven through Standardized Field Sobriety Tests that the National Highway Traffic Safety Administration claims will demonstrate impairment equivalent to a BAC level of no lower than .08. Finally, a DUI conviction can be sustained by evidence of the driver's actions (difficulty locating documentation such as your driver's license, insurance card and registration); poor driving (improper lane usage being the most common of the top twenty "markers" of alcohol impairment); conduct (crying, argumentative, confused, repeating yourself); and physical characteristics (slurred speech, bloodshot eyes, poor balance).
The government may charge you with DUI even if you are taking prescribed medications in the proper amount. Taking medications is not automatically illegal. But if the evidence shows that the medications impaired your driving, you could be convicted of DUI.
Illegal drug impairment requires a different analysis. It is illegal to drive with "any amount' of an illegal drug in your "blood, breath or urine". 625 ILCS 5/11-501
With the recent United States Supreme Court decision in McNeely vs. Missouri, if you do not consent to a blood or urine test, the police would most likely be required to obtain a search warrant. The warrant would issue only upon a showing of probable cause to believe you are under the influence of a drug that can be detected in your blood or urine.
In a similar manner, the police can charge you with DUI drugs if they have evidence that you are actually under the influence, even if they lack a blood or breath test. As is the case with a request for a search warrant for a blood test, the issue will come down to the street cop having to present evidence that you are impaired and, more importantly, that your impairment is due to drugs.
Some prosecutors attempt to prove DUI drug charges in which there is no blood or urine test by propounding Drug Recognition Experts (DRE). These "experts" claim that they can "tell" that someone is under the influence of drugs, even though unlike is the case with alcohol, there is no standardized set of tests for determining drug impairment.
Not just anyone is allowed to take the witness stand and give expert opinions. The speaker and the opinions must meet the test of scientific reliability as set forth in Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), the so-called "Frye standard".
In that particular case, the influential DC Circuit Court of Appeals held that the results of polygraph tests (lie detectors) were not sufficiently reliable to be used in a court of law. To date, no Illinois court has ruled on the admissibility of DRE opinions.
Meanwhile, a Swedish company claims to have invented a simple device to detect twelve types of illegal drugs. At this point, the device has no use in the United States and much legal wrangling will no doubt occur when someone attempts to use it. But devices such as this very well may represent the future.
Major High Court Ruling on Warrantless Blood Draws May 10, 2013, Illinois DUI Lawyer Blawg
What tools are available to the police in Illinois for proving a DUI charge? January 11, 2013, Illinois DUI Lawyer Blawg