"Kick in" is a phrasal verb that means to start working or to take effect. There are several synonyms for this expression, including "trigger," "activate," "initiate," "commence," "begin," and "launch." These words can be used interchangeably with "kick in" depending on the context of the situation. For example, you could say "The new medicine didn't kick in until after an hour," or "The start of the race kicked in a surge of adrenaline." Other possible synonyms include "kick off," "start up," "get underway," and "fire up." Regardless of the synonym used, they all suggest a beginning or a new phase of activity.