An opposite of effective is obviously "ineffective."
However, the direct opposite in some situations is "counterproductive" meaning having the reverse effect of what was intended.
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The additive opposite is, but not the multiplicative opposite.The additive opposite is, but not the multiplicative opposite.The additive opposite is, but not the multiplicative opposite.The additive opposite is, but not the multiplicative opposite.
The opposite of word several is few.
more opposite, most opposite
Couldn't is the opposite of could.
exposit