The purpose of this study is to see if the drug Pennsaid (Diclofenac) can help reduce pain felt by people with chronic nerve pain. The drug will be used 2-4 times each day for 2 weeks.

The U.S. Food and Drug Administration (FDA) has approved Pennsaid to treat osteoarthritis (bone pain), but the FDA has not approved Pennsaid to treat neuropathic (nerve) pain.

The research study will compare Pennsaid to placebo.

The research study will compare Pennsaid to placebo. The placebo looks like Pennsaid, but it doesn't contain any Pennsaid. The investigators use placebos in research to see if the results are due to the study drug or to other reasons. At some time during the study the investigators will give the patient Pennsaid. At another time, the investigators will give the patient placebo. The investigators are using Quantitative Sensory Testing (QST), which is temperature testing before and after using the study drug to see if Pennsaid is helpful in reducing people's nerve pain.

In this test, a small metal plate, about the size of a matchbox, is put on the area where the patient has pain. The plate is connected to a computer that can warm or cool the plate. The patient will use a computer mouse button to tell us when the plate feels warm. The QST machine is approved by the Food and Drug Administration (FDA). It is often used by nerve doctors to see if a person has neuropathy (pain caused by damage to a nerve).

Inclusion Criteria:

1. Subject will be between 18 and 80 years of age.
2. Subject has not been on Pennsaid or other topical non-steroid anti-inflammatory drugs for at least one month.
3. Subject agrees to make no change in his/her current pain medications during the entire study period. This requirement will ensure that valid comparisons of primary and secondary measures can be made before and after the study.
4. Subject has a VAS pain score of 4 or above at the beginning of the study.
5. Subject has had a neuropathic pain condition such as those listed above for at least three months. This requirement is to avoid clinical uncertainty from an unstable pain condition and to minimize the study variation.
6. Female subjects of childbearing age must have a negative urine pregnancy test at the initial visit.

Exclusion Criteria:

1. Subject has documented severe liver or renal disease that will affect the elimination of Pennsaid or is subject to the adverse effect of Pennsaid on these organs. (Renal dysfunction is defined as eGFR < 60. Hepatic dysfunction is defined as LFTs ≥ 3X ULN.)
2. Subject has pending litigation related to the neuropathic pain condition.
3. Subject has active skin lesion or open wound at the site of Pennsaid application (e.g., active shingles with skin lesions).
4. Subject is pregnant or lactating.
5. Subject has scar tissue or sensory deficit at the site of QST.
6. Subject is allergic to diclofenac or has cross-sensitivity to other non-steroid anti-inflammatory drugs.
7. Subject has a positive urine (illicit) drug test.
8. Subjects who experience asthma, urticaria or an allergic type reaction when taking aspirin or NSAIDs.
9. Subjects undergoing coronary artery bypass surgery.
10. Subject with a known history of cardiovascular disease, ulcer, gastrointestinal bleed or impaired renal function.
11. Subjects currently using NSAIDS.

Adverse event data was collected over the length of a subject's participation in the study (5 weeks).

Due to the study design, subjects who withdrew before completing the entire study were not considered for any treatment period that they did not participate in due to drop out or withdrawl. Subjects who completed only part of a treatment period were considered.These numbers may differ from the total number enrolled in the study for this reason.