Too effective 〜すぎる
Grammar Explanation: Too much with 〜すぎる

The relaxing effect of the herbal tea Cocoa drank was too effective, as she fell right to sleep. (Note that (効:き)く is a verb, not an adjective, meaning “to be effective” or “to do its work”.)
Key Points
1効きすぎ means the tea worked too well
- 効
( く means taking effect, and it changes to the stem 効( き before すぎる.
2The sentence drops the final る
- The clipped すぎ! is an exclamation based on すぎる, matching the quick reaction to Cocoa falling asleep.
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