Intrathecal morphine and heroin in humans: six-hour drug levels in spinal fluid and plasma.

Lumbar spinal fluid and plasma concentrations of morphine were measured by radioimmunoassay after intrathecal administration of 1 mg of morphine (n = 13) or heroin (n = 10). Plasma levels of morphine were measured regardless of 'whether heroin or morphine was injected intrathecally, because of the rapid biotransformation of heroin to morphine in plasma. Significant drug concentrations appeared in plasma after intrathecal heroin (peak concentration 47.8 +/- 9.0 nmol/L, time to peak concentration 10 +/- 2.4 min); after intrathecal morphine plasma drug concentrations were significantly lower (8.1 +/- 1.0 nmol/L; P less than 0.002) and significantly later (216 +/- 39 min; P less than 0.002). Elimination half-life of heroin from spinal fluid (43 +/- 5 min) was significantly shorter than for morphine (73 +/- 5 min; P less than 0.02).