Is it unhealthy to use a tooth whitening kit?

No, as long as you a: No, as long as you are supervised by a dentist who examines you first to make sure your teeth and gums are healthy and follow his\her directions. If you buy an over the counter kit or a kit online, you always run the risk of problems.

Not usually: Today's whitening products are safe when used as directed. Follow the directions and you should be ok. If you have sensitivity, you may be putting too much in or leaving in too long. If so, take a day or two off, and try some more.

Perhaps?: The kits are designed to be used for a limited time. Chronic use or overuse beyond the instructions will cause chemical burns to soft tissue and create sensitivity on any exposed root surfaces.