Plasma concentration of morphine after i.m., extradural and intrathecal administration

Seventeen patients received morphine 0.2 mgkg-1 in a 10% dextrose solution i.m. (n = 5), extradurally (n = 6) and intrathecally (n = 6) for pain after operation. Morphine was measured in plasma by radioimmunoassay. Plasma immunoreactive morphine concentration was significantly less after intrathecal administration after i.m. and extradural administration (P less than 0.05) at 2, 10, 15 and 30 min. We conclude that morphine given extradurally has a greater initial rate of vascular absorption than morphine given intrathecally and is similar to that observed after i.m. administration.