The Collaborative International Dictionary of English v.0.48:

Worsen \Wors"en\, v. i.
   To grow or become worse. --De Quincey.
   [1913 Webster]

         Indifferent health, which seemed rather to worsen than
         improve.                                 --Carlyle.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Worsen \Wors"en\, v. t.
   1. To make worse; to deteriorate; to impair.
      [1913 Webster]

            It is apparent that, in the particular point of
            which we have been conversing, their condition is
            greatly worsened.                     --Southey.
      [1913 Webster]

   2. To get the better of; to worst. [R.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

worsen
    v 1: grow worse; "Conditions in the slum worsened" [syn:
         worsen, decline] [ant: ameliorate, better,
         improve, meliorate]
    2: make worse; "This drug aggravates the pain" [syn: worsen,
       aggravate, exacerbate, exasperate] [ant: ameliorate,
       amend, better, improve, meliorate]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "worsen":
   accommodate, adapt, adjust, agent provocateur, aggravate, alter,
   ameliorate, amplify, annoy, augment, be changed, be converted into,
   be renewed, better, blast, blight, bottom out, break, break up,
   build up, change, checker, chop, chop and change, come about,
   come around, come round, convert, corrupt, damage, debase, decline,
   deepen, deform, degenerate, degrade, denature, descend,
   deteriorate, deviate, dilapidate, disimprove, disintegrate,
   diverge, diversify, embitter, endamage, enhance, enlarge,
   exacerbate, exasperate, fall back, fit, flop, foul, get worse,
   grow worse, harm, haul around, heat up, heighten, hot up, humble,
   hurt, impair, improve, increase, injure, intensify, irritate, jibe,
   let down, lower, magnify, make acute, make worse, meliorate,
   mitigate, modify, modulate, mutate, overthrow, provoke, put back,
   qualify, re-create, realign, rebuild, reconstruct, redesign, refit,
   reform, regress, relapse, remake, renew, reshape, restructure,
   retrograde, retrogress, revamp, revive, ring the changes, rot,
   sharpen, shift, shift the scene, shuffle the cards, sicken, sink,
   slacken, slip back, sour, subvert, swerve, tack, taint,
   take a turn, turn, turn aside, turn into, turn the corner,
   turn the scale, turn the tables, turn the tide, turn upside down,
   undergo a change, vary, veer, warp, weaken, work a change