Antonyms of harmful:
advantageous, harmless, innocuous, innocent, favorable, helpful, profitable, serviceable, beneficent, good, invigorating, rejuvenating, useful, beneficial, healthful, life-giving, salutary, wholesome, propitious, curative, healthy, palliative, remedial, salubrious, secure, sound, benignant, noncorrosive, nondestructive, nonfatal, noninfectious, nonlethal, nonpoisonous, nonpolluting, nontoxic, nonvenomous, anodyne, benign, hurtless, inoffensive, safe.
Examples of usage:
  1. It has all the fantastic unreality of Chinese dragons, and, therefore, can in no way be harmful.
    "Oscar Wilde", Leonard Cresswell Ingleby.
  2. Meanwhile, for his many sins, a short repentance in prison will not be harmful to his soul.
    "Dramatic Technique", George Pierce Baker.
  3. For the same reason it would seem necessary in India for the Christian convert to separate himself from everything which is in any way distinctive of Hinduism, quite apart from whether the thing itself is harmful or not.
    "India and the Indians", Edward F. Elwin.