What is the opposite of efficacious?

Need antonyms for efficacious? Here's a list of opposite words from our thesaurus that you can use instead.

Adjective
(of something inanimate or abstract) Opposite of successful in producing a desired or intended result
Adjective
Opposite of reasonable and convincing
Adjective
Opposite of denotes the effectiveness or dominance of a particular action
“Leaders who do not look after the interests of their followers are not only unethical but ineffective.”
Adjective
Opposite of having great impact or influence
“We would end up an impotent and uninfluential nation, just one voice among the many.”
Adjective
Opposite of incapable of going wrong or being misused
Adjective
Opposite of having the power to produce a required effect
“The snake oil sold to me was ineffective in relieving my pain.”
Adjective
Opposite of convincing or effective
Adjective
Opposite of based on, or displaying, common sense
“Of course, the moment I got there it was clear that all my concerns were completely groundless.”
Adjective
Opposite of having, showing, or done with good judgment or sense
“His ideas are obviously foolish, easily disproved, an affront to any reasoning person.”
Adjective
Opposite of being in effective or current operation, action, or force
“The rides have become inoperative since its closure in the early 2000s.”
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