Antonym for ameliorate

exacerbate

From WordNet (r) 3.0 (2006) [wn]:

  exacerbate
      v 1: make worse; "This drug aggravates the pain" [syn: {worsen},
           {aggravate}, {exacerbate}, {exasperate}] [ant:
           {ameliorate}, {amend}, {better}, {improve}, {meliorate}]
      2: exasperate or irritate [syn: {exacerbate}, {exasperate},
         {aggravate}]

I don't think there is an antonym that ends with an -iorate because the words ameliorate and meliorate derive from a single stem (see below). The closest etymological antonym is aggravate

ameliorate comes from French meilleur and ameliorer - the direct French antonym for this word is aggraver meaning 'to make worse'.

After I posted this, I remembered the word!

deleterious

del·e·te·ri·ous adjective \ˌde-lə-ˈtir-ē-əs\
: damaging or harmful

Synonyms
adverse, bad, baleful, baneful, damaging, dangerous, harmful, detrimental, evil, hurtful, ill, injurious, mischievous, nocuous, noxious, pernicious, prejudicial, wicked
Antonyms
anodyne, benign, harmless, hurtless, innocent, innocuous, inoffensive, safe
Related Words
hostile, inimical, unfriendly; contagious, deadly, infectious, infective, pestiferous, pestilent, pestilential, poisonous, venomous; insidious, menacing, ominous, sinister, threatening; hazardous, imperiling (or imperilling), jeopardizing, parlous, perilous, risky, unsafe, unsound; nasty, noisome, unhealthful, unhealthy, unwholesome; destructive, fatal, killer, lethal, malignant, ruinous
Near Antonyms
advantageous, beneficial, useful; favorable, good, propitious; curative, healthful, healthy, helpful, palliative, remedial, salubrious, salutary, wholesome; secure, sound; benignant; noncorrosive, nondestructive, nonfatal, noninfectious, nonlethal, nonpoisonous, nonpolluting, nontoxic, nonvenomous