Predictive Markers of the Effects of Opioid Therapy

Inclusion Criteria: * Pain duration ≥ 3 months. * Minimum baseline pain intensity ≥ 4 on a 0-10 numerical rating scale (over the past week). * Maximum baseline pain intensity \< 9 on a 0-10 numerical rating scale (over the past week). * Prescribed opioid treatment (ATC: N02) * Anticipated to stay on prescribed opioid treatment throughout the study, i.e. \>14 days. * Anticipated to stay on any concomitant non-opioid treatment throughout the study, i.e. \>14 days. * Subject may be male or female, age \>18 years old. * Is willing and able to comply with study procedures as judged by the site investigator. * Subject has voluntarily signed and dated the study-specific informed consent form, approved by an Independent Ethics Committee, after the nature of the study has been explained and the subject has had the opportunity to ask questions. The informed consent form must be signed before any study-specific procedures are performed. Exclusion Criteria: * Has a mental incapacity or language barriers precluding adequate understanding of study procedures. * Is considered by the site investigator unsuitable to participate in the study for any other reason, for instance due to a significant serious underlying condition. * Recently received opioids on a daily basis (within the last 10 weeks). * Current alcohol or substance abuse, according to the site investigator's medical judgement. * Is anticipated to undergo a painful procedure(s) (e.g. surgery) during the study, which can interfere with the experience of the chronic pain condition for which the subject is to receive opioids.