Opposite words for harmful:
innocuous, safe, noncorrosive, beneficial, benign, life-giving, inoffensive, useful, salutary, propitious, serviceable, nonpolluting, rejuvenating, palliative, innocent, good, nondestructive, nonfatal, healthful, helpful, noninfectious, nonlethal, healthy, nontoxic, profitable, sound, nonpoisonous, salubrious, favorable, secure, beneficent, advantageous, hurtless, curative, remedial, anodyne, invigorating, wholesome, nonvenomous, benignant, harmless.
Usage samples:
  1. Water is not itself harmful.
    "Lessons on Soil", E. J. Russell.
  2. A little eaten at the end of a meal is not harmful to the normal person.
    "School and Home Cooking", Carlotta C. Greer.
  3. I find it less harmful and expensive to have my pleasure here.
    "The Story of a Mine", Bret Harte.
  4. But more harmful and more unfortunate than any other word or act was the statement of President Wilson that we might be " too proud to fight."
    "With the French in France and Salonika", Richard Harding Davis.