The topical use of higher concentration (10–20%) benzocaine products applied to the mouth or mucous membranes has been found to be a cause of methemoglobinemia, a disorder in which the amount of oxygen carried by the blood is greatly reduced. This side effect is most common in children under two years of age. As a result, the FDA has stated that benzocaine products should not be used in children under two years of age, unless directed by and supervised by a healthcare professional. In European countries, the contraindication applies to children under 12 years of age. Symptoms of methemoglobinemia usually occur within minutes to hours of applying benzocaine, and can occur upon the first-time use or after additional use.