Alencar 2012.

Study characteristics
Methods	RCT, parallel groups
Participants	171 infants admitted to a referral pediatric hospital in Brazil Inclusion criteria: Infant 0‐28 days of life with an indication for large or medium surgical procedure Exclusion criteria: Infants were excluded if they were discharged, had died or needed a new surgical procedure before completing 72 hours after the initial surgery. Also, neonates with chromosomal syndromes or ambiguous genitalia were excluded.
Interventions	Fentanyl, 1–2 μg/kg/h intravenously Tramadol, 0.1–0.2 mg/kg/h intravenously
Outcomes	Primary: time from the end of the surgical procedure until extubation (hours) Secondary: time to reach 100 ml/kg of enteral feeding (hours); pain evaluation during the first 72 hours after surgery (two pain scales: CRIES and NFCS). Pain scales were applied every 2 hours for the first 24 postoperative hours and every 4 hours for the following 48 hours.
Notes	Authors had nothing to declare.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Random sequence generation not reported Quote: "For randomisation, four opaque envelopes were prepared (male/female infants with major/minor surgeries). Each envelope contained 10 blocks of four patients randomly ordered as ‘fentanyl’ or ‘tramadol’. The central pharmacy performed the randomisation".
Allocation concealment (selection bias)	Unclear risk	Allocation concealment not reported Quote: "For randomisation, four opaque envelopes were prepared (male/female infants with major/minor surgeries). Each envelope contained 10 blocks of four patients randomly ordered as ‘fentanyl’ or ‘tramadol’. The central pharmacy performed the randomisation".
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Though it was not mentioned that the two preparations were indistinguishable, it was plausible (because of the color, opacity etc. of tramadol and fentanyl). Quote: "The phials of fentanyl (50 μg/mL) and tramadol (50 mg/mL) were diluted in 9 mL of normal saline. Tramadol solution was further diluted in 9 mL of normal saline. Therefore, 0.2 mL/h of the solution was equivalent to 1 μg/h of fentanyl and 0.1 mg/h of tramadol."
Blinding of outcome assessment (detection bias) All outcomes	Low risk	The decisions regarding extubation and management of enteral feeding were managed by attending neonatologists who were blind to group allocation of the patients.
Incomplete outcome data (attrition bias) All outcomes	Low risk	All patients seem accounted for.
Selective reporting (reporting bias)	Unclear risk	All planned outcomes reported, however, primary and secondary outcomes were switched (protocol versus manuscript)
Other bias	Unclear risk	Lack of standardization of surgical anesthesia; the study design left to the attending neonatologists the decision to extubate and increase enteral feeding of randomized patients.