Beginning January 1, 2020, you will be able to possess of up to 30 grams of cannabis, 5 grams of cannabis concentrate or 500 milligrams of THC without violating Illinois state law.
Under the law you will have to purchase your cannabis from approved dispensaries. You cannot grow it on your own unless you are a medical marijuana cardholder, and even then, you will be limited to 5 plants. You will not be able to legally purchase marijuana from a street dealer.
You also cannot use marijuana in public places. You will have to use it at home, and even then, discretely. You can’t sit on your porch where neighbors can see you toking.
The change will not effect our current DUI law, which still makes it a crime to drive while either impaired due to cannabis or with 5 nanograms or more of THC in one’s blood or 10 nanograms in any other bodily substance (i.e., urine or saliva). This means that I suggest that you do not drive for several hours after using marijuana, probably not until the next day after a full night’s sleep.
Also, the change in the law does not mean that employers cannot still fire you for failing a drug test showing that you were positive for cannabis. Illinois is an “at will” state, meaning that an employer can fire you “at will” for just about any reason (except when in violation of civil rights or contract).
Finally, this change only affects Illinois law. You could still be arrested and charged by Federal officers. This is most likely to happen while on Federal property, whether a Federal building such as a courthouse, post office, or other governmental building, on Federal land or on the lake by the Coast Guard.
Here are some articles with further information about the new law:
Chicago Sun-Times: https://chicago.suntimes.com/cannabis/2019/5/31/18647868/marijuana-illinois-legalization-where-to-buy-amount
Chicago Tribune: https://www.chicagotribune.com/news/breaking/ct-met-cb-legal-marijuana-illinois-20190531-story.html