Methods	RCT 2-arm parallel group design.
Participants	USA: hospital setting. 93 primiparous women in labour, uncomplicated pregnancy at 37 or more weeks’ gestation and in pain described as moderate or severe
Interventions	Experimental: IM pentazocine 60 mg (N = 38). Control: IM pethidine 100 mg (N = 45).
Outcomes	Primary: pain relief assessed at 1 hr, as good or poor. Secondary: maternal side effects, nausea or vomiting, labour progress. Neonatal Apgar score at 1 and 5 min
Notes	If additional analgesia was required, a second injection could be administered at the discretion of medic. Not stated if IOL onset included
Risk of bias
Bias	Authors’ judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Not reported.
Allocation concealment (selection bias)	Unclear risk	Identical vials with code number but no further information given
Blinding (performance bias and detection bias) Women	Low risk	Identical vials with code number.
Blinding (performance bias and detection bias) Clinical staff	Low risk	Identical vials with code number.
Blinding (performance bias and detection bias) Outcome assessor	Low risk	No-one involved with the immediate care of the woman knew the drug identity
Incomplete outcome data (attrition bias) All outcomes	High risk	83/93 women analysed and reasons for missing data not reported
Selective reporting (reporting bias)	Unclear risk	Unclear.
Other bias	Unclear risk	Unclear how many women randomised to each group and balance at baseline unclear