In-office whitening is much stronger and can only be sold to dental professionals who are trained to apply it.
Generally, over-the-counter products are only designed to remove surface stains, many are not proven safe and therefore not recommended.
There are 2 active ingredients in in-office whitening: carbamide peroxide and hydrogen peroxide.
Carbamide peroxide is 1/3 of the strength of hydrogen peroxide. This is commonly used in patients who already have dentinal sensitivity.
It is not as strong however, the results are still great!
Hydrogen peroxide is much stronger and can only be applied by a dental professional, no OTC product will have a strong concentration of this chemical.
OTC products commonly cause gingival irritation (when a portion of the whitening gel seeps onto the gums) and sensitivity due to improper placement, overfilling trays, or if strip size is too big.