Morphine
An opium alkaloid and full mu-opioid agonist used for severe pain after surgery or injury, cancer pain and pain in end-of-life care.
Drug card
- Class
- Opioid
- How it works
- Activating mu-opioid receptors in the brain and spinal cord to dull pain
- Used for
- Severe pain after surgery or injury
- Cancer pain
- Pain in end-of-life (palliative) care
- Known for
- Constipation
- Slowed, shallow breathing (respiratory depression)
- Pinpoint pupils (miosis)
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does morphine work?
Morphine, an alkaloid of the opium poppy, is a full agonist that prefers the mu receptor but binds delta and kappa receptors at higher doses. Its pain relief has no ceiling; side effects such as breathing depression limit the dose. It releases histamine, which can cause itching, flushing and a fall in blood pressure.
Like other opioid analgesics: Opioids activate mu-opioid receptors, the G protein-coupled receptors used by the body's own opioids in the brain, spinal cord and gut. In nerve cells this opens potassium channels and closes calcium channels, so neurons fire less and release less pain transmitter such as substance P. They also depress the brainstem breathing center, narrow the pupils and slow the gut; tolerance and dependence build with regular use.
Uses
What is morphine used for?
- Severe pain after surgery or injury
- Cancer pain
- Pain in end-of-life (palliative) care
Side effects
Side effects of morphine
- ConstipationKnown for
- Slowed, shallow breathing (respiratory depression)Known forSerious
- Pinpoint pupils (miosis)Known for
- Nausea and vomiting
- Drowsiness and sedation
- Dizziness
- Confusion
- Itching and flushing
- Low blood pressure on standing
- Addiction, misuse and dependenceSerious
- Withdrawal symptoms if stopped suddenly
- Difficulty passing urine
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Contraindicated in significant respiratory depression, acute severe asthma without monitoring, and bowel obstruction.
- Contraindicated with an MAOI, or within two weeks of stopping one.
- In kidney failure its glucuronide metabolites (M3G, M6G) build up; start low and watch for sedation.
- Benzodiazepines, alcohol and other CNS depressants add to its sedation and breathing depression.
Good to know: Unlike tramadol, it releases histamine, which accounts for the itching, flushing and drop in blood pressure it can cause.
Remember it
A way to remember it
MORPHINE
- M
- Miosis (pinpoint pupils)
- O
- Opioid withdrawal if stopped suddenly
- R
- Respiratory depression
- P
- Pruritus (itching)
- H
- Hypotension
- I
- Intestinal slowing: constipation
- N
- Nausea and vomiting
- E
- Euphoria, then dependence and addiction
Written for biofideX
Its class
Opioid analgesics
Easily mixed up with opioid antagonists, benzodiazepines and antimotility drugs.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Morphine sulfate extended-release tablets: label (DailyMed)
- Morphine (NCI Thesaurus)
- Morphine (NHS)
- Tramadol hydrochloride extended-release tablets: label (DailyMed)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Pain, fever and inflammation
Morphine is one of 15 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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