Will expired Crest White Strips still work?

Answer 1

Not nearly as well. The active ingredient is hydrogen peroxide. It degrades to water and oxygen over time. After expiration, there is still active ingredient, but, depending on long they've been expired, the effect is diminished.

http://www.3dwhite.com/teeth-whitening/3d-white-faq.aspx

Answer 2

Yes, they do. Anecdotal evidence: I've noticed a definite improvement, but it only lasts a few months. I asked my dentist recently about all these new strip whiteners and he said they were extremely hard on the gums! So beware! Ask your dentist before using them and you dentist can brighten you smile more safely.