The law sets in motion the process for the sale of licenses to growers and dispensaries, and eventually the sale of marijuana itself on January 1, 2020.
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Supporters have also said the bill will begin to undo the effects of what they call the failed war on drugs.
"Signing this bill into law won't do the injustices of the past, or make full the lives that were interrupted," Pritzker said at the signing. "We can't turn the clock back, but we can turn the page."
The Democratic-controlled general assembly overwhelmingly approved the bill last month.
The bill makes Illinois the second state to legalize the cannabis possession through the legislative process. The signing of the bill also makes Illinois the first to legalize retail sales legislatively.
The bill allows adults 21 and over who are residents of Illinois to purchase up to 30 grams of cannabis flower, edibles totaling 500mg of THC, and five grams of concentrated THC products. Visitors will be able to purchase half those amounts.
Medical marijuana patients will be allowed to cultivate up to five plants at home.
The law also comes with criminal justice reforms and social equity investment programs, including a $30 million low-interest loan program and social equity applicant status for licensing.
The social justice component of the law will provide expungements for an estimated 700,000 people who have been arrested or convicted for possession of 30 grams of pot or less.
Those who have been arrested or convicted of possession of more than 30 grams can petition for expungement on a case-by-case basis.
Dispensaries already selling medical marijuana will get the first recreational licenses, which will cost a minimum of $750,000 each.
Licenses and then tax revenue is expected to raise $57 million the first year.
"But we're reserving licenses for those who come after so we can make sure this industry goes, to a large degree, to small business people, to people who want to enter the industry and to the equity participants we are trying to aim at," Pritzker said.
Marijuana will be packaged in child-resistant containers and have warning labels. Dispensaries will also have to be dispersed at least 1,500 feet from each other.
"Because we don't want to see that liquor store on every corner phenomenon happening," said State Rep. Kelly Cassidy (D-Chicago), bill sponsor. "And those are all floors for locals. Locals can opt out completely, locals can require that these facilities be farther apart."
With 55 medical marijuana dispensaries in place and plenty of product being produced, Cresco Labs is already in position for January 1.
"The scaling up to meet the demands of adults is our focus," said Charlie Bachtell, CEO of Cresco Labs.
Cresco will be doubling the number of dispensaries and likely doubling their workforce as well.
LAW ENFORCEMENT CONCERNED ABOUT HOW TO ENFORCE LEGALIZATION
One group that did not join Pritzker's bill signing ceremony was law enforcement.
"Why enact this bill when the potential for wrongdoing and crime is there and law enforcement can't do anything about it?" asked Chief Steve Stelter, president of the Illinois Association of Chiefs of Police.
Stelter, who serves as police chief in West Chester, said that with only six months until the new law is enacted, there is still nothing in the bill that addresses police concerns about enforcement when it comes to driving under the influence of marijuana. Unlike alcohol, Stelter said the only recourse an accident victim has is a civil suit, not criminal action.
"There is no viable testing that can prove that a subject is under the influence of marijuana," he said.
Beside the lack of viable testing, Stelter also said there are no extra resources for detection recognition training.
Supporters say the new law will be tweaked over time, but is ready to go on January 1.
"This has been around since the last three legislative sessions," Bachtell pointed out. "It's been discussed, there's been a tremendous amount of thought that was compiled in this law."
The law does still allow employers to maintain a zero tolerance policy, meaning they can fire or otherwise discipline anyone who tests positive for marijuana in a drug test. It was not immediately clear if lawmakers will pursue further legislation or an amendment to change that.
You can also get a DUI for driving under the influence of marijuana.
Opponents believe legalization will only lead to increased addiction, drivers who are high and overdoses. They said they're talking to federal law enforcement officials and attorneys to determine their next steps.
Illinois had already decriminalized the possession of small amounts of marijuana but the federal ban remains in place.
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