inefficacious

/ˌɪnɛfɪˈkeɪʃəs/
adjective
  1. Not producing the desired effect; failing to achieve the intended result.
    • Many home remedies for colds are inefficacious according to scientific studies.
    • Despite hours of negotiation, the peace talks were inefficacious and the conflict continued.
    • The old medicine proved inefficacious against the new strain of the virus.