Synonyms for Harmful:

adj.

baneful (adjective)
malefic.
damaging (adjective)
nocent.
dangerous (adjective)
pestilent, Nocuous.
destructive (adjective)
deadly, calamitous, destructive, disastrous, devastating, deleterious, injurious, detrimental, catastrophic, Extirpative, consumptive, damaging.
evil (adjective)
sinister, maleficent.
harmful (adjective)
damaging, detrimental, deleterious, unwholesome, baleful, injurious, malignant, unhealthy.
injurious, hurtful (adjective)
damaging, deleterious, Harassing, corroding, sinister, bad, murderous, Nocuous, cataclysmic, virulent, unhealthy, unwholesome, internecine, malignant, dire, baneful, baleful, adverse, malefic, disastrous, subversive, risky, catastrophic, toxic, ruinous, malicious, inimical, sinful, painful, mischievous, crippling, destructive, menacing, unsafe, disadvantageous, detrimental, incendiary, calamitous, consumptive, corrupting, evil, pernicious.
noisome (adjective)
mephitic.
ruinous (adjective)
ruinous.
unhealthy (adjective)
unwholesome.
wicked (adjective)
wicked.

n.

perilous, risky, hazardous. help. harmful (noun)
defamatory, adverse, calumnious, libellous, deadly, inauspicious, ill, dangerous, denigratory, prejudicial, baneful, hurtful, abusive, corrupting, libelous, untoward, denigrative, counterproductive, insidious, disadvantageous, nocent, damaging, wounding, slanderous, traumatic, vesicatory, bad, ruinous, injurious, unfavorable, catastrophic, negative, deleterious, calumniatory, bruising, stabbing, evil, malign, minus, degrading, subtle, mischievous, pestilent, vesicant, unsafe, detrimental, unfavourable, pernicious, denigrating, prejudicious.
injurious (noun)
fell, habit-forming, incendiary, fatal, vicious, mephitic, ominous, Nocuous, doing harm, serious, ill-omened, felonious, costly, dire, malefic, wild, grievous, stunting, annihilative, Sapping, maleficent, diabolic, internecine, wicked, toxic, corrupt, fraught with harm, venomous, demoniac, Afflicting, murderous, aching, unpropitious, devilish, Fiendish, pricking, brutal, sore, devouring, malicious, inimical, unlucky, subversive, cruel, sinister, corroding, corrosive, crippling, cataclysmic, treacherous, menacing, Sinistrous, sinful, unfortunate, undetermining, morbific, distressing, painful, mortal, demolitionary, Harassing, objectionable, virulent, malevolent, Extirpative, consumptive, fraught with evil, lethal, satanic, malignant.

Other synonyms:

hazardous, cataclysmic, treacherous, perilous, inimical. mortal, serious, fatal. deadly
lethal.
high-priced
costly.
noxious
Nocuous.
vicious
cruel.
Other relevant words:
dire, malefic, sinister, unfortunate, prejudicious, corroding, hurtful, wicked, demolitionary, calumnious, internecine, denigrating, mephitic, morbific, wounding, minus, treacherous, serious, Sapping, calumniatory, cataclysmic, habit-forming, stunting, risky, stabbing, diabolic, abusive, subtle, cruel, objectionable, perilous, defamatory, Fiendish, unfavorable, vesicant, subversive, slanderous, corrupt, fell, baneful, devouring, satanic, vesicatory, Extirpative, annihilative, malignant, undetermining, unpropitious, mortal, ill, hazardous, distressing, unfavourable, malign, libellous, pestilent, painful, evil, sore, crippling, corrupting, inauspicious, untoward, unlucky, costly, pernicious, denigratory, incendiary, denigrative, sinful, devilish, consumptive, degrading, ill-omened, toxic, help, negative, menacing, Sinistrous, murderous, Harassing, bruising, maleficent, felonious, adverse, Nocuous, brutal, libelous, counterproductive, corrosive, malevolent, bad, mischievous, aching, insidious, Afflicting, grievous, demoniac, venomous, prejudicial, unsafe, malicious, dangerous, fatal, ominous, traumatic, pricking, virulent, vicious, inimical, lethal, ruinous, disadvantageous, wild, nocent.

Usage examples for harmful

  1. The dread of loss of sleep has often caused people to take somnifacients which eventually proved ever so much more harmful than would the loss of sleep they were meant to overcome. – Health Through Will Power by James J. Walsh
  2. But let me explain myself: this is a novel view to you, and you are perhaps unable to conceive how or why war was so fatal to the old world, because you see how little harmful it is to the new. – Prince Zaleski by M.P. Shiel