MORPHINE: SIDE EFFECTS-RESPIRATORY
Respiratory depression-Respiratory depression occurs due to a direct effect on the respiratory centres in the brain stem. There is decreased sensitivity to the circulating carbon dioxide level and reduction in both tidal volume and respiratory rate.
The risk of respiratory depression is increased in patients who:

• are at risk of respiratory failure for other reasons
• are given morphine in excessive dosage and/or too frequently
• are opioid naive
• are commenced on parenteral rather than oral therapy
• have only mild pain
• have had their pain acutely relieved by a procedure such as a nerve block

There is no evidence that equianalgesic doses of the various opioid analgesic drugs differ significantly in their potential to cause respiratory depression.
Significant respiratory depression is unlikely to occur with usual doses of morphine except in patients who are at risk of respiratory failure for other reasons. In patients bordering on respiratory failure, opioid analgesics will reduce the response to rising carbon dioxide levels and continued respiration depends on hypoxic drive; administration of oxygen in this situation may be fatal.
In patients with intracranial malignancies, the increased arterial carbon dioxide tension which occurs following the administration of an opioid drug may cause cerebrovascular dilatation and a paradoxical increase in headache.
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Pain Relief/Muscle Relaxers