Cancer pain is one of the most severe symptoms in patients with advanced cancer. Approximately 92% of patients with cancer pain have one or more pain symptoms caused directly by cancer, and about 21% of patients have one or more pain symptoms caused by cancer therapies.

The prevalence of neuropathic cancer pain (NCP) is estimated to be more than 30% in patients with cancer pain. Oral opioids are advocated in several international guidelines as first-line treatment for moderate-to-severe cancer pain. Although opioids can also be used to treat NCP, the response is often low, requiring higher doses.6,7 Furthermore, the quality of life (QOL) of patients with NCP refractory to opioid can be substantially reduced by its side effects, even when the dose is carefully titrated.