Police released a surveillance photo of the suspect.
They say he entered the CVS Pharmacy on Randall Road shortly before 11:00 p.m. on Sunday.
The suspect displayed a handgun and gave a note to the pharmacist demanding the strong narcotic pain reliever, oxycodone.
After he was given the drugs, the man fled in a silver or gray two-door Toyota or Mazda with a rear deck spoiler.
Anyone with information is asked to contact the Crystal Lake Police Department.