What will the effects be if I take plan b while I'm also taking depo?

If you are late on your injection, go ahead with the plan b, but if you are current with your injections you shouldn't need it.

If, for some reason you did happen to use both, it wouldn't likely be an issue, but you probably wouldn't want to be causing even more hormone changes on a regular basis. It is not intended for regular use.

You would possibly have some spotting or bleeding, and/or it may alter your menstrual cycle a bit... causing your period to start a little sooner or a little later than expected.