Pharmacokinetics and analgesic effect of slow-release oral morphine sulphate in volunteers.

Sustained-release oral morphine sulphate (MST) 20 mg was administered to 11 healthy volunteers. Mean peak plasma morphine concentration was 14.8 ng ml-1, and occurred at a mean time of 142.5 min after ingestion. Analgesia assessed by an ischaemic forearm pain test increased to a maximum approximately 40 min after the calculated peak plasma concentration of morphine had been achieved, and remained greater than control values as plasma morphine concentration decayed. However, there was not a significant correlation between analgesia and plasma morphine concentration. This may result from delay in brain penetration by morphine. The mean systemic availability of morphine in the first 7 h after administration of MST was 18.3%.