What is a synonym for kicks in?

There isn't one perfect synonym for "kicks in," as it depends on the context. Here are some options depending on the meaning you want to convey:

If you mean "to start to have an effect":

* Takes effect

* Begins to work

* Comes into play

* Sets in

* Starts working

If you mean "to enter a place or situation forcefully":

* Barges in

* Busts in

* Charges in

* Storms in

* Forces entry

If you mean "to participate in something actively":

* Gets involved

* Joins in

* Participates

* Takes part

* Chimes in

Please provide more context, and I can give you a more specific synonym. For example, "The medicine finally kicked in," or "He kicked in the door and stormed inside."