The state Senate approved a measure last May and the House may vote on it soon.
The bill would start a three-year pilot program allowing people who suffer from certain debilitating illnesses to have up to six cannabis plants.
Police warn legalization will only lead to more crime.
"The passing of this bill is going to lead to more crime and drug use. Street gangs will open marijuana dispensaries and they'll use the profits for selling that marijuana to buy guns and drugs and bail out fellow gang members," said Phil Cline, former Chicago police superintendent.
Fourteen states currently have laws permitting medical marijuana.
The Illinois House sponsor of the bill says it may come up for a vote by Thursday.