Inclusion criteria for opioid group

• Age > 18 years old
• ASA 1-3
• Chronic pain, defined as persisting pain for more than three months
• Daily use of more than 60 mg of morphine equivalent in more than 7 days
• Legally competent and able to give informed consent

Inclusion criteria for the opioid naive control group

• Age > 18 years old
• ASA 1-3
• No use of opioids within 30 days prior to participation
• Legally competent and able to give informed consent

Exclusion Criteria:

• Age > 84 years old
• Volunteers who cannot cooperate with the study
• Volunteers who cannot understand or speak Danish
• Allergy to the lidocaine or the preservatives used in the study
• Alcohol or drug abuse other than opioids
• Peripheral neuropathy at the upper limb non-dominant arm
• Pathology or previous major surgery to the upper limb
• Active signs of infection in the cutaneous area of injection
• Pregnancy at the time of the trial
• Partial or complete 2nd or 3rd degree atrioventricular heart block without a pacemaker
• Severe liver disease
• Severe kidney disease and reduced kidney function (eGFR below 30 mL/min)
• Daily treatment with drugs that are structurally related to local analgesics or class IB antiarrhythmics, as the toxic effects are additive
• Daily treatment with class III antiarrhythmics e.g., amiodarone.