The term 'opioid' is a generic term for naturally occurring, semisynthetic and synthetic drugs which combine with opioid receptors to produce physiological effects and which are stereospecifically antagonised by naloxone. For clinical purposes, opioids can be classified according to their receptor interactions (agonist, partial agonist, agonist-antagonist and antagonist), the pain intensity for which they are conventionally used (moderate or severe), and their half-life (short or long). Pure agonists conventionally used for moderate pain, short and long half-life pure agonists conventionally used for severe pain, mixed agonist-antagonists and partial agonist opioids are described in detail. The effective clinical use of opioid drugs requires familiarity with drug selection, routes of administration, dosage guidelines and potential adverse effects. Opioids are unequivocally indicated in the management of severe acute pain and moderate to severe pain associated with cancer. There is increasing acceptance of the role of opioids in the management of recurring acute pain, chronic nonmalignant pain of organic origin and severe neuropathic pain. The selection of opioids is influenced by pain intensity, pharmacokinetic and formulary considerations, previous adverse effects and the presence of coexisting disease. Some patients will require sequential trials of several different opioids before a drug which is effective and well tolerated is identified. Opioid agents should be administered by the most comfortable and convenient route that meets the specific needs of the patient. The regimen for opioid medications should generally provide around-the-clock analgesia with provision for rescue doses for the management of exacerbations of the pain not covered by the regular dosage. At all times, uncontrolled pain should be addressed by gradual increase in the opioid dose until either pain control is achieved or intolerable and unmanageable adverse effects supervene. The management of pain with opioid analgesics demands frequent patient assessment and a readiness to re-evaluate the therapeutic plan in the setting of either inadequate relief or adverse effects.