Prilocaine is metabolized to products that include o-toluidine, an agent capable of converting hemoglobin to methemoglobin. Though tolerated in healthy persons, even moderate methemoglobinemia can cause decompensation in patients with cardiac or pulmonary disease. The ester-type local anesthetics are metabolized to products that can cause antibody formation in some patients. Allergic responses to local anesthetics are rare and can usually be prevented by using an agent from the amide subclass. In high concentrations, local anesthetics may cause a local neurotoxic action (especially important in the spinal cord) that includes histologic damage and permanent impairment of function.