Opioid analgesics in palliative care

The discovery of the opioid receptors began in the 1970s and three (μ, δ and κ) are now recognised. Most current opioid analgesics interact preferentially with μ receptors, although some have actions at δ and κ receptors. The revelation that opioid receptors are present on peripheral sensory neurones and immune system cells, has led to the realisation that the opioid system is involved in a host of actions, including the central and peripheral modulation of pain transmission and the regulation of immune and neuroendocrine processes. Opioid pharmacology has become even more complex with the recent suggestion of multiple receptor subtypes (eg, μ1 and μ2).The predominant subtype can have a profound effect on analgesic response. In addition, the existence of subtypes may partially explain incomplete cross-tolerance to μ receptor opioids and the need to reduce the equianalgesic dose by 25–30 per cent when switching between them. The effects mediated by each receptor type are shown in Panel 1.