Effective January 1, 2014, Illinois legalized the medical use of marijuana. In order to do so, it became necessary to change Illinois DUI law since driving with any amount of marijuana in a person's system is illegal and may subject the person to a DUI charge. Under the new law, if the person holds a registered user's permit from the Illinois Department of Public Health, that person may drive with marijuana in his or here blood or urine as long as he or she is not impaired.
However there is a significant trade-off for the right to drive after using marijuana. The police may require the registered user to submit to physical performance tests. If the person refuses or fails testing then he or she may be suspended. Non-registered users have no obligation to submit to such testing and do not face suspension for refusing or failing testing.
Attorney Larry A. Davis' article on the subject was published in the Illinois Bar Journal in March 2014. The full text is available here.