Oral ketamine is increasingly used by doctors to both reduce the amount of narcotic pain medication consumed and to improve chronic pain management in difficult cases. How much is absorbed when taken orally, as opposed to intravenously ,is unknown. Ketamine may cause sedation, and occasionally cognitive impairment. Therefore, there are safety concerns associated with its usage as an out-patient may engage in activities where cognitive impairment is dangerous,such as driving. This study is designed to measure the plasma concentrations of ketamine and its active metabolite, norketamine, in people taking low dose oral ketamine for chronic pain. The goal of the study is to verify that low dose oral ketamine produces plasma concentrations below the level associated with toxic symptoms,and thus can be safely used long-term for the treatment of chronic pain .