The Collaborative International Dictionary of English v.0.48:

debilitative \debilitative\ adj.
   causing weakness. [Narrower terms: debilitating (vs.
   invigorating)]

   Syn: enervating, enfeebling, weakening.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

debilitative
    adj 1: causing debilitation [syn: debilitative, enervating,
           enfeebling, weakening]