expression, ichidan verb
to work too well (of a punishment, reprimand, etc.); to be too much (for someone); to have a stronger effect than intended
Idiomatic expression used when a punishment, scolding, or harsh measure has an excessive effect, often implying it backfires or causes unintended suffering. The 'medicine' metaphor suggests the remedy is too strong.
彼を叱ったら、薬が効きすぎて落ち込んでしまった。
When I scolded him, it worked too well and he got depressed.
I meant it as a light warning, but it had a stronger effect than intended and she burst into tears.