The Collaborative International Dictionary of English v.0.48:

Unhealthy \Unhealthy\
   See healthy.

WordNet (r) 3.0 (2006):

unhealthy
    adj 1: not in or exhibiting good health in body or mind;
           "unhealthy ulcers" [ant: healthy]
    2: detrimental to health [syn: insalubrious, unhealthful,
       unhealthy]
    3: not conducive to good health; "an unhealthy diet of fast
       foods"; "an unhealthy climate"

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "unhealthy":
   ailing, bad, bad for, cachectic, chancy, contaminated, corrupt,
   damaging, dangerous, debilitated, degenerate, depraved,
   destructive, detrimental, diseased, doubtful, drained, dubious,
   enervated, exhausted, failing, feeble, flagitious, foul, frail,
   hairy, harmful, hazardous, healthless, ill, in poor health,
   indisposed, infirm, injurious, insalubrious, insanitary, insecure,
   invalid, jeopardous, languishing, malign, morbid, morbific,
   moribund, nefarious, noisome, noxious, pale, pathogenic,
   pathological, peaked, peaky, peccant, perilous, perverse,
   pestiferous, polluted, putrid, reduced, reduced in health, risky,
   rocky, rotten, run-down, septic, shaky, sick, sickly, tainted,
   tottery, touch-and-go, treacherous, uncertain, undependable,
   unhealthful, unhygienic, unpredictable, unreliable, unsafe,
   unsanitary, unsound, unstable, unsteady, unsure, untrustworthy,
   unwell, unwholesome, valetudinarian, valetudinary, vicious,
   villainous, weakened, weakly, wicked, with low resistance