Dentists use two types of numbing medications, including a topical numbing agent, which is applied to the area, and an injectable anesthetic that provides longer-lasting results, states Wikipedia. Sometimes the two are used together, with the first medication numbing the injection site prior to the shot.
The injection is performed to block sensation to particular nerves that are affected by dental work, most commonly the inferior alveolar nerve, according to DentalCare.com, produced by the Procter & Gamble Co. The most common injectable numbing agent is lidocaine.
One of the most common topical numbing agents is benzocaine, according to Wikipedia. Benzocaine also is found in some over-the-counter cough drops and in ointments for mouth ulcers.