Illinois has one of the toughest laws in the nation as it relates to driving while under the influence of drugs. A person is deemed guilty of DUI when he or she drives: (1) "under the influence of any...drug or combination of drugs to a degree that renders the person incapable of safely driving" or (2) "there is any amount of a drug, substance, or compound in the person's breath, blood, or urine resulting from the unlawful use or consumption of cannabis [marijuana] . . . a controlled substance . . . an intoxicating compound . . . or methamphetamine." Note that being actually impaired does not seem to be required under part (2) of the offense, only the presence of the drug in the person's system.
Further, the statute states that "The fact that [the driver] . . . is or has been legally entitled to use [any] drug . . . or intoxicating compound . . . shall not constitute a defense." This seems to indicate that the fact that you have prescription for a drug will not be an affirmative defense.
Recent court decisions have tried to provide some clarity to these issues.
In 2011, in People v. Martin, the state Supreme Court ruled that a driver found with the slightest amount of illegal drugs in his or her body can be convicted of a felony when their driving was the cause of a fatal crash. Proof of impairment is not required, which means that a prosecutor no longer has to prove that the drug itself contributed to the crash, only that the bad driving did. The driver in this particular case had taken a small amount of methamphetamine.
Up to that point in time, prosecutors had always believed that they were required to show impairment for fatal crashes involving drugs. Commenting on this case, as quoted by the Chicago Tribune, an associate professor of pharmacology said, "God forbid you take your spouse's Vicoden because you happen to have a toothache [and] get into an accident[.] . . . This puts Illinois . . . at the bottom of the heap for law enforcement and the courts using responsible science."
In 2012, in People v. Vente, the state Appellate Court ruled the statutory provision that legal entitlement to use drugs does not constitute a defense to a charge of driving under the influence will not prevent a driver from lawfully using prescription medications, as long as such use does not render the driver incapable of driving safely. In Vente, the defendant was charged with unsafe driving while under the influence of drugs, and driving while there was codeine and morphine in her system, resulting from the unlawful use or consumption of a controlled substance. The evidence showed that the defendant had consumed morphine and codeine and that the drug's presence was due to the use of a validly prescribed cough medicine. Therefore, the presence of controlled substances in her urine was not the result of "unlawful use or consumption" and, further, there was little evidence of unsafe driving.
Looking at the statutory and case law, the results seem to be that, when you have a controlled substance in your system, impairment does not need to be proved. However, if you have a valid prescription, and if you are driving safely, then you cannot be convicted of DUI. If you are ever arrested for drug DUI, you should contact an experienced DUI attorney, who will investigate the facts of your case as they relate to the existing law.