Background: Long-term pain is a debilitating condition that is costly to treat and has a significant impact on patient quality of life. Classical opioids have been used for the treatment of pain for centuries and are one of the most effective drug classes available for acute severe pain and long-term pain. However, concerns regarding adverse effects, tolerance to analgesic effects and the potential for addiction have resulted in a reluctance to prescribe and use opioids for the management of long-term non-cancer pain. Adverse events, including gastrointestinal side effects such as constipation, nausea and vomiting, and central nervous system side effects such as sedation are responsible for as many as one in five patients discontinuing opioid treatment, often leading to inadequate pain relief and poor patient quality of life. Therefore, new analgesic therapies are needed that are associated with fewer adverse effects, whilst providing sustainable pain relief for patients with long-term pain.