You bought a box of Whitestrips to brighten your teeth for a wedding that never happened. Can you still use them if the box has expired? That depends on what result you are seeking. You might be wasting your time if you want bright and beautiful teeth.
You do not need to be overly concerned about safety with using expired Whitestrips. They will not hurt your teeth. Professional grade Whitestrips do contain water though and beauty products containing water can eventually breed bacteria.
Once expired, the Whitestrips may not be as effective according to Crest Whitestrips review's website. This means that teeth may not get as white as they would have if the strips were used before the expiration date. However, Whitestrips are only effective on certain types of stains, so before using the product, read the instructions.
Expiration dates on beauty products are not regulated by the Food and Drug Administration unless they contain active drugs. So they are guesses at best. You can look for signs that products have gone bad like a strange smell, damaged packaging, or box discoloration. According to the Cosmetics Cop website, any beauty product containing water will expire more quickly than one with dry ingredients. Whitestrips do contain purified water.