Donepezil Compared to Placebo in Patients With Chronic Neuropathic Pain

Inclusion Criteria: * Diagnosis of diabetic neuropathy or failed back syndrome with neuropathic symptoms * Age 18-80 * Taking a stable dose of gabapentin or pregabalin Exclusion Criteria: * Pregnant women or women of child-bearing potential not willing to practice a reliable form of birth control * Allergy to donepezil or other piperidine derivatives (including fentanyl, alfentanil, sulfentanil, remifentanyl, demerol, tramadol, loperamide, diphenoxylate, betaprodine, alphaprodine, ethopropazine, anileridine, piminodine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP),loratadine, fexofenadine * Unstable medical conditions including cardiac, pulmonary, renal or hepatic diseases that, in the opinion of the investigator, would preclude patients from finishing the trial * Any person with pending litigation * A history of major psychosis requiring hospitalization within the last three years * Non-English speaking, illiterate, unable to comprehend consent * Lack of contact information * Uncontrolled narrow-angle glaucoma * Currently being treatment with thioridazine (Mellaril) * Patients taking opioids will be excluded if they are taking a dosage that exceeds an equivalent of 30 mg of morphine per day * Patients taking more than one regular (not rescue) medication for pain * Patients taking donepezil for dementia * Patients with a baseline pain score less than 2 (0-10 scale) or greater than 8 (0-10) will be excluded