As a rule you should not leave the circle until all your spell-work and any other rituals are finished and the circle has been closed or dispersed but that does not mean that you can’t or that there will be detrimental effects if you do. There won’t be provided that you do it properly. Whatever the reason is for needing to exit temporarily it can be done.
The first thing you need to do is to stand in the east and literally cut a “doorway” with the athame through which you can walk. Once the doorway is cut simply walk through it taking the athame with you. Once you are through the doorway you should turn and face the circle and then using the athame again you should close the opening. Doing so prevents negativity or other forces from entering your sacred area while you are gone. Nothing can enter or leave.
Once you have finished your call of nature (or whatever it was you needed to do) you need to basically repeat the procedure to get back in. Don’t be tempted to just walk into the circle but once again use your athame to open a doorway and then close it.