Opposite words for beneficial:
damaging, unprofitable, deleterious, unhelpful, injurious, harmful, unfriendly, disadvantageous, insalubrious, bad, unfavorable.
Usage samples:
  1. Even those beneficial exercises upon which he may have stumbled, become of little practical value; because, when the principles upon which they are based are unknown, they can neither be followed up with certainty, nor be varied without danger.
    "A Practical Enquiry into the Philosophy of Education", James Gall.
  2. " You advised me to come here because it would be novel as well as beneficial," she wrote.
    "Louisiana", Frances Hodgson Burnett.
  3. In view of the probable beneficial effects of that law, I recommend that the policy established by it be maintained.
    "Complete State of the Union Addresses from 1790 to the Present", Various.
  4. The influence of his genial kindness was never forgotten by his youngest brother; but there was a stronger and more dominating personality of which the effect was less beneficial to a sensitive and nervous child.
    "The Life of Froude", Herbert Paul.