analgesics: morphine

Written by Chloe Doris

Last Reviewed: September 2019

Review Due: September 2020


mechanism of action2

Morphine belongs to a group of drugs known as opioids – they exert their effect through direct G-protein coupling to the ion channels on neuronal membranes in the CNS and GI tract. 

They promote the opening of potassium channels and inhibit the opening of voltage-gated calcium channels. These membrane effects decrease neuronal excitability and reduce neurotransmitter release – the overall effect is therefore inhibitory.


Pain (acute and chronic)

Myocardial infarction

Acute pulmonary oedema

Palliative care (cough, pain, dyspnoea at rest)


side effects1

Respiratory depression

Dependence or withdrawal



Constipation, GI discomfort, reduced appetite

Dizziness, drowsiness, dry mouth






Acute respiratory depression

Comatose patients

Head injury

Raised intracranial pressure


dose adjustments1

Hepatic Impairment

Should be cautious. Avoid oral administration in acute impairment.

Renal Impairment

Avoid use or reduce dose. Opioid effects increased and prolonged.



Alcohol causes rapid release of morphine.

Buprenorphine and pentazocine are predicted to increase the risk of opiate withdrawal when given with morphine.

Many drugs can have CNS depressant effects which might affect ability to perform skilled tasks.


Drowsiness may affect performance of skilled tasks and the effects of alcohol will be enhanced. Driving should be avoided at the start of therapy and following any dose changes.


Repeated use of Morphine is associated with the development of psychological and physical dependence – caution is prescribing to patients with a history of drug abuse.



  1. British National Formulary. Morphine. (2019)

  2. Rang H.P, Ritter J.M, Flower R.J, Henderson G. Rang & Dale’s Pharmacology. 8th Ed. Elsevier Churchill Livingstone; 2016. Page 519.

  3. British National Formulary. Interactions: Morphine. (2019)