Is peroxide safe to use to whiten teeth?

should I use it straight or with water?
--------------------
You can use hydrogen peroxide straight as a rinse to whiten your teeth (it's mostly water already). My grandma has done it for years, it's an old fashioned home remedy. In fact, the listerine whitening pre-brushing rinse is mostly hydrogen peroxide! Make sure not to swallow it, though. And be aware that it can be very drying... I don't use the hydrogen peroxide myself, but the listerine rinse made my mouth peel if I used it too much.
Source