The Collaborative International Dictionary of English v.0.48:

Exacerbate \Ex*ac"er*bate\ ([e^]gz*[a^]s"[~e]r*b[=a]t; 277), v.
   t. [imp. & p. p. Exacerbated
   ([e^]gz*[a^]s"[~e]r*b[=a]`t[e^]d); p. pr. & vb. n.
   Exacerbating ([e^]gz*[a^]s"[~e]r*b[=a]`t[i^]ng).] [L.
   exacerbatus, p. p. of exacerbare; ex out (intens.) +
   acerbare. See Acerbate.]
   To render more violent or bitter; to irritate; to exasperate;
   to imbitter, as passions or disease. --Brougham.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exacerbate
    v 1: make worse; "This drug aggravates the pain" [syn: worsen,
         aggravate, exacerbate, exasperate] [ant:
         ameliorate, amend, better, improve, meliorate]
    2: exasperate or irritate [syn: exacerbate, exasperate,
       aggravate]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "exacerbate":
   accelerate, acerbate, agent provocateur, aggravate, alienate,
   amplify, annoy, antagonize, augment, beef up, bitter, blow up,
   build up, chafe, complicate, concentrate, condense, consolidate,
   damage, deepen, deteriorate, dilapidate, double, embitter,
   endamage, enhance, enlarge, envenom, estrange, exaggerate,
   exasperate, feed the fire, fret, gall, grate, grate on, gravel,
   grit, harm, heat up, heighten, hop up, hot up, hurt, impair,
   increase, inflame, infuriate, injure, intensify, irritate, jazz up,
   key up, madden, magnify, make acute, make complex, make worse,
   provoke, put back, ramify, rasp, redouble, reinforce, set against,
   set at odds, set on edge, sharpen, soup up, sour, step up,
   strengthen, triple, weaken, whet, worsen