Additional Study Information: Peripheral Neuropathy is a result of nerve damage, which may cause weakness, numbness, tingling, and pain in your hands, feet, and other areas of your body. Over 20 million Americans are affected by peripheral neuropathy, most commonly caused by diabetes. In approximately 25-50% of all people with peripheral neuropathy, the cause is unknown. These cases may be diagnosed as cryptogenic sensory polyneuropathy (CSPN). CSPN symptoms may become worse with heat, fatigue, and physical activity. Currently, there are no medications approved for the treatment of CSPN by the U.S. Food and Drug Administration (FDA). Some medications are used off-label (not approved by the FDA) to treat CSPN, and they may not be covered by patients insurance. The costs of these medications can be financially burdensome to patients; therefore, there is a need to identify medications that can be used to treat CSPN. There are four drugs that will be tested in this study: nortriptyline, duloxetine, pregabalin and mexiletine. These four drugs were created for the treatment of other, unrelated diseases or problems. These four drugs seem to help patients with pain, but we are not sure which drug is the best and has the fewest side effects. By doing this study, researchers hope to learn about the safety and effectiveness of nortriptyline, duloxetine, pregabalin and mexiletine in treating CSPN.