Think of whitening gel that you buy from a Dentist as prescription strength gel with all the safety measures in place. Think of over the counter gels as a marketing idea to get you to buy them with no regards for the safety of your teeth and gums. Don't do your own root canal and don't do your own teeth whitening.
Are All Teeth Whitening Gels OK or Can Some Ruin Your Teeth?
Doctor Answers 2
Use ADA Seal approved whitening gels for safety
There are some whitening gels on the market that have a low pH (the gels are acidic). These gels can hurt your teeth. Make sure your whitening products have the ADA Seal of approval -- or use a professional whitening system (by consulting with your dentist).