Pharmacodynamics/Pharmacokinetics

Relative potency of morphine compared to fentanyl is 1:100 (i.e. fentanyl 0.1 mg equivalent to morphine 10 mg) in adults[2]. There is one randomised, controlled trial comparing the continuous infusion of fentanyl (10.5 microgram/kg for 1 hour followed by 1.5 microgram/kg/hour) versus morphine (140 microgram/kg for 1 hour followed by 20 microgram/kg/hour) in newborn infants undergoing mechanical ventilation which revealed equivalent analgesic effect with fewer side effects for fentanyl (21). The relative potency of fentanyl from this study in newborns compared to morphine is estimated to be 13 to 20:1 [22]. There is no study directly comparing the potency of fentanyl to morphine in newborns. (LOE II GOR B)

Estimated oral morphine bioavailability 48.5% in neonates [1]. (LOE IV GOR C)

In adults, morphine’s elimination half-life is similar for the intravenous, intramuscular, subcutaneous and oral routes of administration [13].

Effective morphine concentrations in the range of 10–20 ng/mL have been reported [14, 15]. Concentrations above 20 nanogram/mL have been associated with respiratory depression [16].

The mean morphine half-life is age related, reported as around 9 hours in ventilated preterm infants [17, 18], 6 hours in term infants [18, 19] and 2 hours for infants beyond 11 days age [18].

Stability: Ethanol-free morphine 2 mg/mL oral solution diluted to 0.4 mg/mL with sterile water and stored in a light protected container at room temperature retained 107% of its original concentration after 60 days [20].