Illinois is joining 19 other states around the country that have signed off on the controversial treatment.
Jim Champion of west suburban Somonauk has lived with multiple sclerosis for 25 years.
"At one time I took 59 pills a day, and I was a zombie," said Champion. "I would fall asleep in the middle of conversations."
The only treatment that he says has helped is smoking marijuana.
"You could see in my legs that the muscles had stopped moving for the first time in days, and it brought such relief," said Champion.
For Champion, 47, it is a watershed day as Illinois joins 19 other states and the District of Columbia in legalizing pot for medical use.
"I can tell you that I've seen people on their death beds asking me: 'Please get this done. Please get this done,'" said Paul Bachmann of Americans for Disabled Americans.
The law will start as a four-year pilot program. Patients and caregivers will have to undergo background checks. Marijuana dispensaries will be regulated by the state. Patients will only be allowed to buy up to two-and-a-half ounces at a time.
"I say this bill has more controls in it than any bill ever written, not only on marijuana but on drugs in general," said State Rep. Lou Lang (D-Skokie).
"We've seen abuse in other states, particularly in California, so there's a lot of restrictions in there to pacify the more conservative, cautious members of the legislature," said ABC7 political analyst Laura Washington.
Opponents of the new law say those restrictions are not enough to prevent people from obtaining the drug for recreational use. Several law enforcement groups, including the Chicago Crime Commission, say the law will have unintended consequences.
"The abuse is built-in, because the law permits people to drive right after smoking marijuana," said former Drug Enforcement Agency Administrator Peter Bensinger. "So that's a danger not only to the smoker, but to every Illinois citizen on the highways."
Other opponents of the measure include the Illinois Sheriffs' Association and Mothers Against Drunk Driving.