Am I insane? I've checked everywhere in the drugstore and the back room and I can't find the key. Do I have to progress the story before I can find Frank's medicine? --- "Pretend...I'm saying...something profound..." - Takeda Shingen, Samurai Warriors II
You have to go to the drive in motel to save Glenn with Carley. There you get an axe. When you go back to the drug store talk to Doug. The two of you will walk outside to look around behind the gate. Show Doug the picture of Lee's family to get him to agree to opening the gate. Break the lock with the axe, pick up the brick, use the tv remote from the office desk on the store across the street, and then the brick on the window.
After that you just walk over to the other side of the street and get the key.
END SPOILERS --- Blood! Sometimes it sets my teeth on edge. Other times it helps me control the chaos. -Dexter Morgan