Pharmacokinetics
Absorption: Variably absorbed (about 30%) following oral administration. More reliably absorbed from rectal, subcut, and IM sites. Following epidural administration, systemic absorption and absorption into the intrathecal space via the meninges occurs.
Distribution: Widely distributed. Crosses the placenta; enters breast milk in small amounts.
Protein Binding: Premature infants: <20%; Adults: 35%.
Metabolism and Excretion: Mostly metabolized by the liver. Active metabolites excreted renally.
Half-life: Premature neonates: 10–20 hr; Neonates: 7.6 hr; Infants 1–3 mo: 6.2 hr; Children 6 mo–2.5 yr: 2.9 hr; Children 3–6 yr: 1–2 hr; Children 6–19 yr with sickle cell disease: 1.3 hr; Adults: 2–4 hr.
TIME/ACTION PROFILE (analgesia)
ROUTE ONSET PEAK DURATION
PO unknown 60 min 4–5 hr
PO-ER unknown 3–4 hr 8–24 hr
IM 10–30 min 30–60 min 4–5 hr
Subcut 20 min 50–90 min 4–5 hr
Rect unknown 20–60 min 3–7 hr
IV rapid 20 min 4–5 hr
Epidural 6–30 min 1 hr up to 24 hr
IT rapid (min) unknown up to 24 hr