How dangerous is it to use homemade hydrogen peroxide and baking soda mix to whiten teeth?

Not very dangerous.: There is no great danger. In fact it was a combination recommended by gum specialists years ago to help fight gum disease. I do not believe it will be very effective in whitening your teeth, however. Baking soda doesn't whiten and 3% hydrogen peroxide is very weak compared to the manufactured whitening gels.

Not effective: You are wasting your time. If it was that easy and that effective, there would be absolutely no need for professional whitening products.. Everyone would just make their own concoction. If you want a great looking smile, see your dentist.

Safe...but: Safe but not effective. Don't waste your time.

Benign: Tastes horrible but ok to use. Hydrogen peroxide will bubble in mouth. Can help with periodontal bacteria. Not curative but high oxygen tension can slow advance.

Don't swallow it: Long before it was in toothpastes, patients were using this combination. It is effective in cleaning, however ther is no Fluoride being provided to the teeth. If you like using this, i would recommend a Fluoride rinse or toothpaste afterwards. Happy cleaning.