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.