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]