Biological consequences of long-term intrathecal administration of opioids.

The intrathecal administration of opioids produces a powerful analgesia through the activation of the spinal opioid receptors. The long term administration of opioids by this route is a valid technique for the treatment of chronic pain of malignant or non-malignant origin. Little is known about the effects of opioids administered by spinal route on various body systems which are not purely tied up to the nociception. It is known that exogenous opioids can interact with their receptors outside the classical nociceptive system. In this context, opioids can modulate the activity of various biological systems such as the immune and the endocrine one. The knowledge of the effects of opioids on these systems is of primary clinical importance. The modulation of the biological systems by exogenous opioids modifies the homeostasis of the body and the clinician should be aware of these modifications in order to be able to anticipate them, to monitor or to use them to improve the therapeutic plan. The knowledge of the influence of the administration route on the variability of these modifications is also important. A survey of the most important knowledge on this topic is presented.