Conjugated are prepared from antinociceptive a agents, particularly opioids or opioid analoges, more particularly deltorphins, enkephalins or analogs thereof, by combining said antinociceptive agent with a material providing a functionally reactive group capable of reacting with a blood component (preferably cell or protein). Said conjugates permit extension of the therapeutic life of the antinociceptive agent. They may be administered to patients to alleviate pain, produce analgesic effects, or assist in cases of narcotics withdrawal, and may also be used as probes for receptor activity. The administration to the patient ma be made either in vivo or ex vivo and may be performed by either introducing the derivative including the reactive functional group into the patient's vascular system or preparing such a conjugate externally (or in vitro) and introducing that conjugate to the patient's vascular system.