WordNet (r) 3.0 (2006):

overreact
    v 1: show an exaggerated response to something; "Don't overreact
         to the bad news--take it easy"

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "overreact":
   aggrandize, amplify, ballyhoo, build up, burlesque, caricature,
   carry too far, draw the longbow, exaggerate, exhaust, go overboard,
   go to extremes, go too far, hyperbolize, idealize, know no bounds,
   lay it on, magnify, make much of, overact, overappraise,
   overassess, overcalculate, overcarry, overcharge, overcount,
   overdevelop, overdo, overdraw, overdrive, overelaborate,
   overemphasize, overesteem, overestimate, overexercise, overexert,
   overexpend, overindulge, overlabor, overmeasure, overplay,
   overpraise, overprize, overrate, overreach, overreact to,
   overreckon, oversell, overspeak, overspend, overstate, overstrain,
   overstress, overstudy, overtask, overtax, overtrain, overuse,
   overvalue, overwork, pass all bounds, pile it on, protest too much,
   puff, stretch, stretch the truth, talk big, talk in superlatives,
   tout, travesty