Every whitening product on the market uses one of two active ingredients: hydrogen peroxide or carbamide peroxide, which breaks down into hydrogen peroxide. What changes between an $8 kit at the drugstore and a $400 course from a dentist is concentration, how the gel is held against the teeth, and who is watching.
Concentration
OTC whitening strips in the United States typically deliver hydrogen peroxide in the 6 to 14 percent range. Dentist-dispensed take-home gels commonly use 10, 15, or 22 percent carbamide peroxide (roughly one-third that concentration expressed as hydrogen peroxide). In-office gels applied under isolation can reach 25 to 40 percent hydrogen peroxide.
Delivery and fit
Custom trays made from an impression of your teeth keep gel in contact with enamel and away from soft tissue. Boil-and-bite trays sold at the drugstore are looser, which both dilutes the active gel with saliva and increases the risk of gum irritation.
Strips solve the fit problem for the front six to eight teeth but do not reach premolars or molars, which is a limitation for anyone with a broad smile line.
Oversight
The ADA's professional guidance emphasizes a pre-treatment exam to screen for cavities, exposed root surfaces, defective restorations, and cracks - all of which can turn a routine whitening course into a painful one. This is the variable an OTC purchase cannot replicate.
How to choose
For mild extrinsic staining in an otherwise healthy adult mouth, an ADA-Accepted OTC product is a reasonable first step. For deeper color change, tetracycline staining, single dark teeth, or any history of sensitivity or gum recession, a dentist-supervised course is the safer and usually more predictable option.