Opioids can be classified according to their mode of synthesis into alkaloids, semi-synthetic and synthetic compounds.There are three classical receptors (DOP, KOP and MOP). The novel NOP receptor is considered to be a non-opioid branch of the opioid receptor family.Opioids can either act as agonists, antagonists or partial agonists at these receptors.Opioid agonists bind to G-protein coupled receptors to cause cellular hyperpolarisation.MOP receptor agonists act in the central and peripheral nervous system to elicit analgesia.