Lyric discussion by PureSophist 

Cover art for Teenage World lyrics by Mountain Goats, The

From Wikipedia: Dextroamphetamine is a powerful psychostimulant which produces increased wakefulness, energy and self-confidence in association with decreased fatigue and appetite. (Dexedrine is a brand name of dextroamphetamine.)

Make of it what you will, because I tried to figure this one out but I can't find any meaning. ;-)

Very late to reply to this, but I think the reason he says, "my teeth hurt" is because stimulants like Dexedrine sometimes make your jaw clench and make you grind your teeth.