US Pharm. 2009;34(3)(Oncology suppl):14. 

The FDA issued a Public Health Advisory warning about possible serious and life-threatening side effects from improper use of topical anesthetics, skin-numbing products available OTC and by prescription. Topical anesthetics, which contain drugs such as lidocaine, tetracaine, benzocaine, and prilocaine in a cream, ointment, or gel, relieve pain by desensitizing nerve endings. However, the drugs can be absorbed into the bloodstream and, if used improperly, may cause life-threatening side effects such as irregular heartbeat, seizures, breathing difficulties, coma, or even death.

The FDA has received reports of adverse events and the deaths of two women who used topical anesthetics before laser hair removal, and two years ago the agency issued an advisory warning of life-threatening effects of these products for cosmetic procedures. A small study reported recently in Radiology found no serious side effects in women using a lidocaine gel covered in plastic wrap during mammography, but the agency is concerned about the use of topical anesthetics during mammography as a result of the serious side effects associated with the drugs' use in laser hair removal.

The agency recommends that patients use topical anesthetics sparingly and consider using a product with the lowest amount of medication. The FDA also advises against a heavy application of topical anesthetics over large areas of skin, applying the products to broken or irritated skin, wrapping topical anesthetic-treated skin with plastic wrap or other dressings, or applying heat from a heating pad to topical anesthetic-treated skin.