Side effects include brilliance...

I've had this one written down for months but have yet to post it.

-

Guy (to teacher): My head is really hurting. Could I go to the nurse and get a Tylenol?
Teacher: Sure, just let me call the office to make sure she's there.
Guy: Okay.
[Teacher makes call.]
Teacher: They say the nurse isn't there... so they can't give you Tylenol. They can give you Tums, though. Do you still want to go?
Guy: Sure.
[Guy leaves classroom, is gone about fifteen minutes, and returns. He takes his seat.]
Guy (to friend beside him): Hey... Hey! Hey, [name]! What do Tums do to you?
Friend: What? Oh, I don't know. Why?
Guy: I just had three.