Which of the following is NOT an over-the-counter tooth whitening product?

In-office bleaching is not an over-the-counter tooth whitening product. This method is performed by dental professionals in a controlled environment, allowing them to use higher concentrations of bleaching agents that are not available to the general public. The process typically involves a more advanced application technique, sometimes using light or heat to enhance the whitening effects, which requires a dental professional's expertise to ensure patient safety and achieve optimal results.

On the other hand, products such as brush-on whitening, whitening strips, and trays with bleach gels are designed for consumer use and can be purchased without a prescription. These products are formulated with lower concentrations of whitening agents, making them safe for at-home use while still providing some degree of whitening efficacy.