ILLINOIS — Thursday at the University of Chicago Center for Care and Discovery, Gov. Pat Quinn signed the state’s medical marijuana bill into law, with Illinois joining 19 other states and the District of Columbia that have legalized cannabis for patient use.

The law, which sets up a four-year trial program, doesn’t take effect until Jan. 1. After that, state regulators will spend months coming up with rules spelling out who gets to open the 22 marijuana growing operations across Illinois, which businesses get the 60 licenses to sell the finished product and how patients will get medical marijuana cards from doctors. Then a panel of state lawmakers will decide whether the regulations are drawn tightly enough.

By the time that plays out, crops are grown and the pot stores open, it could be fall 2014 before those afflicted with one of 40 or so medical conditions can legally toke up to ease their pain.

But the long lead time is not stopping the legalized marijuana industry from descending on Chicago in three weeks for a “cannabusiness symposium.”

The decision by Illinois leaders to take the plunge on medical marijuana after decades of resistance will unfold amid concerns that stretch from law enforcement officials who fear unused pot will be sold illegally to anti-drug educators who worry about the mixed message the law sends to kids to suburban zoning boards who’d rather pot shops not pop up on Main Street.

There will be plenty of time for such issues to be aired.