Methods	RCT 3-arm parallel group design.
Participants	Setting: Thailand - hospital. 135 women. 37 to 42 weeks’ gestation, cx ≥ 3 cm, inactive labour and requiring analgesia Parity: not reported.
Interventions	Experimental: IM tramadol 100 mg (N = 45); IM morphine 100 mg (N = 45). Control: IM pethidine 100 mg (N = 45). Second injection possible after 1 hr of half original study dose, each participant could receive maximum of 2 doses
Outcomes	Maternal outcomes: pain severity/relief 30 min, 1, 2, 3, and 4 hrs (rated good, satisfactory, no response), drowsiness, nausea, vomiting. Neonatal outcomes: Apgar at 1 and 5 min, neonatal resuscitation
Notes
Risk of bias
Bias	Authors’ judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Not reported.
Allocation concealment (selection bias)	Unclear risk	Not reported.
Blinding (performance bias and detection bias) Women	Unclear risk	States blind but does not describe the method.
Blinding (performance bias and detection bias) Clinical staff	Unclear risk	States blind but does not describe the method.
Blinding (performance bias and detection bias) Outcome assessor	Low risk	Medical students unaware of group allocation assessed outcome
Incomplete outcome data (attrition bias) All outcomes	Low risk	All participants analysed.
Selective reporting (reporting bias)	Unclear risk	Unclear.
Other bias	Low risk	Age and maternal weight balanced at baseline.