The Collaborative International Dictionary of English v.0.48:

Antidote \An"ti*dote\ ([a^]n"t[i^]*d[=o]t), n. [L. antidotum,
   Gr. 'anti`doton (sc. fa`rmakon), fr. 'anti`dotos given
   against; 'anti` against + dido`nai to give: cf. F. antidote.
   See Dose, n.]
   1. A remedy to counteract the effects of poison, or of
      anything noxious taken into the stomach; -- used with
      against, for, or to; as, an antidote against, for, or to,
      poison.
      [1913 Webster]

   2. Whatever tends to prevent mischievous effects, or to
      counteract evil which something else might produce.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Antidote \An"ti*dote\, v. t.
   1. To counteract or prevent the effects of, by giving or
      taking an antidote.
      [1913 Webster]

            Nor could Alexander himself . . . antidote . . . the
            poisonous draught, when it had once got into his
            veins.                                --South.
      [1913 Webster]

   2. To fortify or preserve by an antidote.
      [1913 Webster]

WordNet (r) 3.0 (2006):

antidote
    n 1: a remedy that stops or controls the effects of a poison
         [syn: antidote, counterpoison]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "antidote":
   alexipharmic, antacid, antiserum, antitoxin, antivenin, backfire,
   buffer, corrective, counteractant, counteractive, counteragent,
   counterirritant, countermeasure, counterpoison, counterstep, cure,
   drug, medicament, medication, medicine, neutralizer, nullifier,
   offset, preventative, preventive, prophylactic, remedy, specific,
   theriac, theriaca