The purpose of this research study is to measure how much, if any, ketamine is absorbed into the blood stream after ketamine gel is applied to the skin. The investigators expect that the topical administration will provide pain relief locally, at the site of pain, but not be absorbed into the bloodstream and thus not cause side effects. This research will help assess the safety of this drug by measuring the blood concentrations of the drug.

Ketamine is a general anesthetic drug but also has excellent pain relieving qualities. It has been used to relieve chronic pain by administering intravenously, by mouth, or as an injection beneath the skin. When given these ways ketamine can occasionally cause side effects like dizziness, nausea, nightmares, agitation, hallucinations. Recently it has been used topically for patients with neuropathic pain in order to avoid the dizziness and nausea side effects.

Neuropathic Pain can be partially caused by the misfiring of small nerve fibers close to the area of pain. By applying it on the skin, it is expected the drug can penetrate the skin and act directly on the small nerve fibers. The advantage is that less drug will get into the blood circulation. Up to now, it has not been carefully studied how much of the drug appears in the circulation after application on the skin.

Inclusion Criteria:

• Patients with peripheral, focal neuropathic pain such as or complex regional pain syndrome following surgery, bony, soft tissue trauma or nerve trauma, associated with significant allodynia and hyperalgesia who score equal to or greater than 4 in the DN4 questionnaire.
• Duration of pain more than 3 months.
• Ability to speak English adequately to consent to and participate in the study

Exclusion Criteria:

• Allergy to ketamine
• Severe medical illnesses like, e.g. unstable angina, tachyarryhthmias, renal or hepatic failure
• History of psychosis.
• Patients who are already on oral ketamine treatment.
• Patients who are taking HIV Antiretrovirals: (indinavir, nelfinavir, ritonavir saquinavir); Antibiotics: (clairithromycin, itraconazole, ketoconazole, telithromycin,fluconazole, erythromycin); Calcium Channel Blockers (verapamil, diltiazem),Amiodarone, Ciprofloxacin. (These drugs inhibit CYP 3A4 enzyme which metabolises ketamine)

Patients attending the St Josephs Hospital pain clinic with chronic neuropathic pain.