Revisiting old friends: update on opioid pharmacology

Opioids are commonly prescribed for pain due to malignant and non-malignant diseases. They are effective, but have potentially fatal toxicities. Opioid analgesics act as agonists at the mu opioid receptor. Some products combine a mu agonist and antagonist, but there are limitations to their use. Genetic variations may explain why people respond differently to opioids. Some patients have an inadequate response to codeine because they poorly metabolise it to morphine. Switching from one opioid to another is sometimes necessary, but must be done carefully. Use conversion tables as a reference, but be aware of their limitations.