"Kicks in" is a popular phrase that refers to a sudden onset of an event or sensation. It can be substituted with quite a few synonyms to enliven everyday speech or writing. Some common synonyms for "kicks in" are: starts, begins, takes effect, comes on, sets in, dawns, floods, triggers, activates, and initiates. These substitutes have various degrees of formality, which means they can be used in different contexts depending on the tone of the conversation or writing. Whatever synonym one chooses, it is essential to use it appropriately and clearly to convey the intended meaning.