Recent experimental and clinical research has demonstrated that opioid
s can produce potent and receptor-specific analgesic effects outside t
he central nervous system. Opioid receptors are localized on periphera
l terminals of sensory nerves and their endogenous ligands, opioid pep
tides, have been discovered in immune cells within inflamed tissue. Ma
ny controlled clinical studies confirm the experimental data. These fi
ndings open up the exciting perspective of the development of an entir
ely novel generation of peripherally active opioid analgesics devoid o
f centrally mediated side effects.