Morphine Pharmacology

NewsGuard 100/100 Score

Morphine structure and chemistry

Morphine is an opiate found in opium, the juice secreted by the seedpods of poppies. It is a potent pain reliever and is similar in structure to other opiate analgesics.

Mechanism of action

Opioid receptors

As morphine binds to opioid receptors, molecular signalling activates the receptors to mediate certain actions.

There are three important classes of opioid receptors and these are:

  • μ receptor or Mu receptors - There are three subtypes of this receptor, the μ1, μ2 and μ3 receptors. Present in the brainstem and the thalamus, activation of these receptors can result in pain relief, sedation and euphoria as well as respiratory depression, constipation and physical dependence.
  • κ receptor or kappa receptor - This receptor is present in the limbic system, part of the forebrain called the diencephalon, the brain stem and spinal cord. Activation of this receptor causes pain relief, sedation, loss of breath and dependence.
  • δ receptor or delta - This receptor is widely distributed in the brain and also present in the spinal cord and digestive tract. Stimulation of this receptor leads to analgesic as well as antidepressant effects but may also cause respiratory depression.


Morphine can be administered orally, intravenously, rectally, subcutaneously, through spinal injection (e.g. epidural) as well as through inhalation or snorting. The drug has a significant amount of first-pass metabolism in the liver with only around 40 to 50% of the amount absorbed actually reaching the nervous system. Most of the morphine is processed in the kidneys and eliminated from the body in urine.

Adverse effects or side effects of morphine

Morphine has many side effects. Some of the more common and more dangerous ones include:

  • Nausea, vomiting and abdominal cramps
  • Constipation
  • Sedation and drowsiness
  • Itching and allergic skin reactions causing warmth and flushing
  • Shrinking of the pupils to pin points
  • Respiratory depression or suppressed breathing
  • Initial doses lead to euphoria but higher doses cause unpleasant symptoms such as hallucinations, delirium, dizziness and confusion
  • Formation of physical or psychological dependence and development of withdrawal symptoms when use of the drug is stopped
  • Development of tolerance and the need to increase dose to achieve the same degree of effects as before
  • Risk of overdose and poisoning
  • Transmission of HIV/AIDS and hepatitis B and C among needle users.

Morphine dose

Morphine sulphate used for pain relief may be injected intramuscularly in doses of 100-150 mcg/kg every 2 hours as required. For operations and anesthesia, the dose of morphine that needs to be administered depends on the duration of surgery and the severity of pain anticipated.

Further Reading

Last Updated: Jun 19, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Mandal, Ananya. (2023, June 19). Morphine Pharmacology. News-Medical. Retrieved on April 17, 2024 from

  • MLA

    Mandal, Ananya. "Morphine Pharmacology". News-Medical. 17 April 2024. <>.

  • Chicago

    Mandal, Ananya. "Morphine Pharmacology". News-Medical. (accessed April 17, 2024).

  • Harvard

    Mandal, Ananya. 2023. Morphine Pharmacology. News-Medical, viewed 17 April 2024,


The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
Post a new comment

While we only use edited and approved content for Azthena answers, it may on occasions provide incorrect responses. Please confirm any data provided with the related suppliers or authors. We do not provide medical advice, if you search for medical information you must always consult a medical professional before acting on any information provided.

Your questions, but not your email details will be shared with OpenAI and retained for 30 days in accordance with their privacy principles.

Please do not ask questions that use sensitive or confidential information.

Read the full Terms & Conditions.