Outcome	Measurement tool	Measurement occasions
Primary outcome
Pain severity	Pain Severity Score of the Brief Pain Inventory	Baseline, weeks 2, 4, 6 and 12
Secondary outcomes
Physical functioning (generic)	Pain Interference Score of the Brief Pain Inventory.	Baseline, weeks 2, 4, 6 and 12
Physical functioning (condition-specific)	Roland-Morris Disability Questionnaire (24 items, for participants reporting low back pain only) and Neck Disability Questionnaire (for participants reporting neck pain only)	Baseline and 6 weeks
Time to recovery (average daily pain of 0 or 1 of 10 for the past seven consecutive days)	Pain diary	Daily until recovery or up to 12 weeks
Quality of life (physical)	SF-12	Baseline, weeks 2, 4, 6 and 12
Quality of life (mental)	SF-12	Baseline, weeks 2, 4, 6 and 12
Participants’ rating of global improvement	Global Perceived Effect scale	Baseline, weeks 2, 4, 6 and 12
Other outcomes
Adverse events	Self-report and doctor report	Self-report at weeks 2, 4, 6 and 12 Doctor report after each follow-up visit
Work absenteeism	Self-report	Baseline, weeks 2, 4, 6 and 12
Use of treatment or health care services	Self-report	Baseline, weeks 2, 4, 6 and 12
Compliance to study medication	Self-reported adherence recorded in a medication diary compared against doctor prescription data, supported by returned medicine count	Medicine diary is recorded daily over 6 weeks Doctor prescription data at each visit Returned medicine count at end of treatment period (≤6 weeks)
Success of blinding	Participants are asked to estimate their allocation group as active opioid, inactive placebo or do not know	Week 6
Long-term outcomes
Pain severity	Pain Severity Score of the Brief Pain Inventory	Weeks 26 and 52
Use of treatment or health care services	Self-report	Weeks 26 and 52 if still experiencing low back pain and/or neck pain (> 1/10)
Risk of misuse	Current Opioid Misuse Measure	Weeks 12, 26 and 52