1. Table 1 ‐ Overview of included studies.

Study ID	Country	Sample Size	GA (weeks)	Intervention	Comparison
Comparison 1 ‐ Different doses of the same opioid
None
Comparison 2 ‐ Different routes of administration of the same opioid
None
Comparison 3 ‐ Continuous infusion versus bolus administration
Bouwmeester 2001	The Netherlands	68	Not described	Continuous morphine	Intermittent morphine
Bouwmeester 2003a	The Netherlands	63	381	Continuous morphine	Intermittent morphine
Bouwmeester 2003b	The Netherlands	68	36.1 to 42.1	Continuous morphine	Intermittent morphine
Lynn 2000	United States of America	83	Not described	Continuous morphine	Intermittent morphine
Van Dijk 2002	The Netherlands	181	Not described	Continuous morphine	Intermittent morphine
Vaughn 1996	United States of America	16 and 202	39.8 to 42.43	Continuous fentanyl	Intermittent fentanyl
Comparison 4 ‐ 'As needed' administration versus 'as scheduled' administration
Czarnecki 2020	United States of America	25	36.9 to 39.5	Parent/nurse‐controlled analgesia (morphine)	Continuous infusion (morphine)

GA stands for "Gestational Age"; ID stands for "Identification".

1. We extracted the GA for the said 'Group 1' showed in the study, which included newborn infants.
2. The study had two clearly separate phases: In phase 1, 16 newborn infants were enrolled following randomization; in phase 2, 20 newborn infants were enrolled, without randomization, and therefore their outcome data were not included in this review.
3. Inserted data reported the range of GA for the study for both groups, in both study's phases.