WordNet (r) 3.0 (2006):

side effect
    n 1: a secondary and usually adverse effect of a drug or
         therapy; "severe headaches are one of the side effects of
         the drug"
    2: any adverse and unwanted secondary effect; "a strategy to
       contain the fallout from the accounting scandal" [syn: side
       effect, fallout]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "side effect":
   accession, accessory, accompaniment, addenda, addendum, additament,
   addition, additive, additory, additum, adjunct, adjuvant,
   afterbirth, afterclap, aftercrop, aftereffect, afterglow,
   aftergrowth, afterimage, aftermath, afterpain, aftertaste, annex,
   annexation, appanage, appendage, appendant, appurtenance,
   appurtenant, attachment, augment, augmentation, by-product, coda,
   complement, concomitant, continuation, corollary, effect,
   extension, extrapolation, fixture, increase, increment, offshoot,
   pendant, placenta, reaction, reinforcement, response, secundines,
   side issue, supplement, tailpiece, undergirding