noun
Refers to a counter-effect or backlash, often in scientific, technical, or figurative contexts where an action produces an opposing result.
この薬の逆作用として眠気が出ることがあります。
As a reaction to this medicine, drowsiness may occur.
新しい政策の逆作用で、かえって混乱が
As an adverse effect of the new policy, confusion spread instead.
副作用 specifically means 'side effect', especially of medicine. 逆作用 is broader and can refer to any counterproductive result.
Compound of 逆 (reverse, opposite) and 作用 (action, effect). The exact historical derivation is uncertain, but the word follows standard Sino-Japanese compounding patterns.