A review article on fentanyl summarizes the literature on this drug in neonates. Fentanyl, a synthetic opioid, acts by agonist binding to the mu and kappa opiod receptors. It is metabolized by CYP3A4 in the liver. It provides rapid analgesia, blocks endocrine stress responses and prevents pain-induced increases in pulmonary vascular resistance. It is 50-100 times more potent than morphine on a weigh basis. It causes less histamine release than morphine and causes less hemodynamic instability. The pharmacokinetics vary widely – for example the described half-life in various studies varied twelve-fold. Muscle rigidity can occur with high doses, possibly due to modulation of GABA pathways and can manifest as chest wall rigidity and laryngospasm. These are reversible with naloxone (10 micrograms per kilogram). Tolerance to fentanyl can develop rapidly, especially with infusion. High doses of fentanyl can cause neuroexcitation and rarely, seizure-like activity. Respiratory depression can occur with doses above five micrograms per kilogram and it may also occur unexpectedly because of redistribution.