The Collaborative International Dictionary of English v.0.48:

agonizing \agonizing\ adj.
   causing agony. Opposite to painless.

   Syn: excruciating, harrowing, torturing, torturous,
        torturesome.
        [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

Agonize \Ag"o*nize\, v. i. [imp. & p. p. Agonized; p. pr. &
   vb. n. Agonizing.] [F. agoniser, LL. agonizare, fr. Gr. ?.
   See Agony.]
   1. To writhe with agony; to suffer violent anguish.
      [1913 Webster]

            To smart and agonize at every pore.   --Pope.
      [1913 Webster]

   2. To struggle; to wrestle; to strive desperately.
      [1913 Webster]

WordNet (r) 3.0 (2006):

agonizing
    adj 1: extremely painful [syn: agonizing, agonising,
           excruciating, harrowing, torturing, torturous,
           torturesome]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "agonizing":
   acute, afflictive, atrocious, biting, consuming, cramping, cruel,
   desolating, distressful, distressing, excruciating, exquisite,
   fierce, gnawing, grave, griping, hard, harrowing, harsh,
   heartbreaking, heartrending, heartsickening, heartwounding,
   hurtful, hurting, intense, painful, paroxysmal, piercing, poignant,
   pungent, racking, rending, severe, sharp, shooting, spasmatic,
   spasmic, spasmodic, stabbing, stinging, tearing, tormented,
   tormenting, tortured, torturing, torturous, vehement, violent