I was gonna use some white strips to whiten my teeth but the box says they expired a year ago.?

Yes they are safe to use, but they may have lost some of their potency. This is one reason why manufacturers put expiration dates on products....but the product may be just as effective right now as it was before the expiration date. You'd have to try it and see. It's not going to hurt you

They are perfectly safe to use. You are not goint to digest them, are you? If anything, the affect of the teeht whitening will be a little bit less, but probably not even that.