The Collaborative International Dictionary of English v.0.48:

Drastic \Dras"tic\, a. [Gr. ?, fr. ? to do, act: cf. F.
   drastique. See Drama.] (Med.)
   Acting rapidly and violently; efficacious; powerful; --
   opposed to bland; as, drastic purgatives. -- n. (Med.) A
   violent purgative. See Cathartic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

drastic
    adj 1: forceful and extreme and rigorous; "drastic measures"

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "drastic":
   Draconian, acute, cutting, desperate, dire, excessive, exorbitant,
   extravagant, extreme, fierce, forceful, furious, great, harsh,
   immoderate, intemperate, intense, keen, outrageous, piercing,
   potent, powerful, puissant, radical, rigorous, rough, severe,
   sharp, splitting, strong, tough, unconscionable, vehement,
   venomous, vigorous, violent, virulent