Strong opioids, usually morphine, may also be given by a patient controlled analgesia (PCA) system. This allows 1-2 mg (whichever is preset by the anaesthetist) to be given on demand with a 5 min lockout period.

The lockout period may be set by the anaesthetist and is to prevent overdose, which carries the risk of respiratory depression. Close and charted observation is required if PCA is prescribed.