This is the drug Bart Simpson is prescribed when he is diagnosed with having attention deficit disorder (ADD), everything works fine until the side effects kick in... After they realise the cause of his strange behaviour (stealing tanks and blowing up National Baseball Spying Satellites).
This is (in my opinion) meant to satirize the prescription of Ritalin, and also the overmedication of hyperactive children in general (shout to QXZ).
This is one of my favourite episodes and also caused one of the best quotes I know of:
Bart: I’m having some side effects from the dope,
Marge: It’s not dope; it’s something to help you concentrate.
Bart: All I know is that my testicles won’t fit in my underwear (we see the two large bulges down the front of his shorts).
Well, maybe it’s better when seen for yourself, but it still cracks me up every time I see it.