Objective To systematically review available paediatric literature on comparisons between morphine (Mo) and hydromorphone (Hm), to guide clinicians to rationally use these medications.

Design Systematic review within four databases for all studies published from 1963 to July 2019.

Setting All paediatric settings.

Eligibility All studies comparing Mo to Hm in individuals younger than 21 years.

Main outcome measures The primary outcome was to compare clinical efficacy and side effects of Mo and Hm. The secondary outcomes were the comparison of pharmacokinetic profiles and the description of predefined Mo to Hm conversion ratios used across the paediatric literature.

Results Among 754 abstracts reviewed, 59 full-text articles met inclusion criteria and 24 studies were included in the analysis: 4 studies compared pharmacodynamics of Mo and Hm and 20 studies reported the use of a predefined Mo to Hm conversion ratio. Most studies had a poor methodological quality. Available evidence suggests that, when given intravenously, the equianalgesic ratio of Mo to Hm is 5:1. Intravenous administration with this ratio results in a similar rate of adverse effects, including pruritus and nausea. The epidural administration with a ratio of 10:1 results in more pruritus and urinary retention with Mo than Hm. Pharmacokinetic data were reported in only one study. A wide range of pre-established ratios for different routes of administration were reported, but few were based on evidence.

Conclusion Current literature does not permit a rational choice between Mo and Hm. A ratio of 5:1 seems adequate for intravenous administration and leads to a similar rate of adverse effects.