Safety and Efficacy Study of Ethosuximide for the Treatment of Complex Regional Pain Syndrome (CRPS)

Inclusion Criteria: * Male or female, age ≥18 years old; * Diagnosis of Complex Regional Pain Syndrome (CRPS) using International Association for the Study of Pain criteria \>6 months; * Normal liver function (AST level \<3x normal level); * Normal kidney function (serum creatinine \<133µmol/L); * Full blood count, haematocrit \>38%; * Willing and able to give informed consent and of completing study questionnaires; * Stable (no change in past two months) but suboptimal pain pharmacotherapy (i.e. additional pain control felt by patient and physician to be necessary); * Able to attend research centre according to the visit schedule; * Women of child-bearing potential must be using a reliable form of contraception i.e. oral contraceptives, a barrier method (condom or diaphragm), intra-uterine device or abstinence. Exclusion Criteria: * Optimal response to opioids, antidepressants, anticonvulsants or anti- inflammatory medications; * Any history or indication of kidney or liver disease; * Any history of alcohol abuse; * Presence of diabetes; * Subjects taking other anti-epileptic drugs, including gabapentin, pregabalin, topiramate, phenytoin, carbamazepine, and oxcarbazepine; * Pregnancy (a serum bHCG pregnancy test will be performed as part of the initial blood panel); * Known or suspected allergy to succinimides, ethosuximide, methsuximide (Celontin®), phensuximide; * Any history of mental illness or disorder, which in the investigators opinion, interferes with the subjects ability to accurately report treatment response; * Participation in other clinical trial in the 30 days prior to enrolment.