1. Primary characteristics of included trials.

Study ID	N***	Included age	Intervention (s)	Comparator (s)
Alencar 2012	171	0 to 28 days	Tramadol	Fentanyl
Ceelie 2013	74	Post‐conceptual age of 36 1/7 week or older to 1 year of age	Continuous morphine	Intermittent paracetamol
Eissa 2021*	15	Patients under 2 years old	Tapentadol	Placebo
Olischar 2014**	71	Neonates born ≥ 32 weeks postmenstrual age	Tramadol	Placebo

*This trial was composed of three different trials, of which only the third trial was a randomized controlled trial: Trial 1 included neonates from birth to < 2 years and considered only pharmacokinetic, safety and tolerability, and exploratory efficacy analyses; trial 2 enrolled preterm neonates (≥ 24 weeks gestational age) to < 2 years; and trial 3 enrolled neonates from birth to < 2 years old and considered efficacy and safety analyses, immediate rescue design with an alternative efficacy endpoint, and subgrouped patients for < 2 years' assessments. For additional information regarding the trial's differences, please, check the full text.

**This trial aimed to assess whether tramadol’s addition to standard analgesia resulted in earlier extubation or reduced analgesic/sedative requirements in postsurgical neonates. All neonates received morphine and 6‐hourly IV acetaminophen. 

***The number of patients for each study is the number of infants that were randomized in the trial. In Alencar 2012 and Ceelie 2013, some infants were excluded after the randomization and thus not included in the analyses; therefore, the N used in our analyses may be different from that in the table above.