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To begin to impact someone or something. How long until these pills take effect? My back is killing me! This piece of legislation takes effect next week.
1 have the intended result: It will be some time before the painkillers take effect.
2 (formal) start to be valid: Your promotion takes effect from the end of the month.
1. To become operative, as under law or regulation: The curfew takes effect at midnight.
2. To produce the desired reaction: The antibiotics at last began to take effect.