Can I put a 125 amp breaker in a200 amp panel

Please explain what you are trying to do. 125 as a main breaker or to run a sub panel?

Start by checking if your panel will accept a 125 Amp breaker, not all panels will accept them.

so the answer is maybe....
what make of panel? GE? Siemens?

If it accepts the Breaker, then sure. As mentioned above

Some calculating may need to be done. What are some of the current Loads? Without computing loads, worse that would happen is 200 Amp would blow. I answered a little too fast in first post. Can you provide a list of the current loads, and loads to be on the 125 amp panel. Thanks