Introduction: Opioids are widely used for the management of cancer and chronic non-cancer pain and the maintenance management of patients with a history of substance misuse. Increasingly the use of opioids is being scrutinised as patients are prescribed opioids for longer periods and the long-term effects of the opioids becomes clinically more relevant and evident. Our work has explored the prevalence of opioid-related side-effects in patients who are prescribed opioids and explored the clinically relevant phenomenon of opioid-induced hyperalgesia. . Methods: Patients were recruited who were prescribed opioids for the management of cancer and non-cancer pain or substance misuse. Quantitative data was collected to explore the prevalence and severity of opioid related side-effects, the impact of opioids on cognitive function and the effect of opioids on peripheral nerve function through quantitative sensory testing. Testing the sensory processing of patients who are on opioids has revealed altered thermal thresholds and the presence of wind-up at non-painful sites indicating central sensitisation. Qualitative description was used to explore the patient experience of an episode of opioid toxicity. Results: Patients have a significant burden of side-effects which have often not been recognised by clinicians. Using the Addenbrooke’s Cognitive Examination much more cognitive impairment has been revealed than has previously been recognised. Altered thermal thresholds and wind-up at non-painful sites suggests altered pain processing as a result of opioids. Themes from the qualitative description highlighted the coping strategies patients’ develop when managing with significant side-effects and toxicity, the covert self-management of their pain and the need to exert control. One of the most significant findings from the qualitative research was the finding of altered sensation and pain description associated with other features of opioid toxicity. Conclusions: The impact of opioids on the cognitive function of patients has significant implications in terms of patients’ involvement in decision-making and functioning in everyday life. The qualitative data reflects the burden of side effects and the descriptions of patients suggest that opioid-induced hyperalgesia exists as part of the spectrum of opioid toxicity. This finding may help physicians identify patients who are developing opioid-induced hyperalgesia and allow them to intervene earlier with a proactive approach.