The four-year trial program, to take effect Jan. 1, will limit use of medical cannabis to treatment of 42 named ailments or diseases, including cancer, multiple sclerosis, AIDS and amyotrophic lateral sclerosis, a motor neuron malady also known as Lou Gehrig's disease.
Quinn signed the measure at the University of Chicago medical school Center for Care and Discovery where he was joined by Jim Champion, an Illinois military veteran who suffers from multiple sclerosis. As the legislation gained momentum, after languishing for years in the Legislature, Quinn cited an encounter with Champion, who maintained medical cannabis gave him relief from war wounds, the Chicago Tribune said.
Champion said he will be glad to be able to get pain relief without breaking the law, the newspaper reported.
"Our goal from the beginning was to provide a better quality of life for some very sick people in Illinois," Rep. Louis Lang, a Democrat from Skokie, a Chicago suburb, and the bill's chief sponsor told the Tribune.
He said the governor's signature on the bill would be "a signal to many people that the state of Illinois still has a good deal of compassion, a good deal of concern for those of us, under a doctor's care, who wish to try a new type of therapy ... to simply feel better."
Some lawmakers have expressed fears the herbal therapy would become a gateway to more serious drug use.
The bill -- which passed the Illinois House 61-57 in April and cleared the Senate 35-21 in May -- will let people purchase and possess as much as 2.5 ounces of pot every two weeks. The prescribing doctor would have to have a longstanding, enduring medical relationship with the patient.
Patients would be allowed to buy the marijuana from 60 dispensing centers statewide and not be allowed to grow their own. Twenty-two growers would be authorized.
Users, growers and sellers would have to undergo criminal background checks. Dispensaries would be under round-the-clock camera surveillance.
Employers and landlords could bar its use in their workplaces and buildings.
Users stopped by police while driving would have to undergo field sobriety tests that could result in loss of driving privileges and the right to use the medical cannabis.
Nineteen other U.S. states and the District of Columbia allow medical marijuana.