In the nervous system
How it works
Binds mainly to mu opioid receptors in the brain and spinal cord, blocking pain — and slowing breathing.
The short version
Morphine docks onto opioid receptors — mainly the mu (μ) receptors — in the brain and spinal cord. That turns down pain signals. It can also cause drowsiness and a sense of well-being (euphoria), and it slows breathing and gut movement.
A little more detail
- It is a full opioid agonist, relatively selective for the mu receptor. At higher doses it can also bind delta and kappa receptors.
- There is no ceiling on pain relief: higher doses keep working. That is also why overdose is so dangerous.
- The same receptor family that eases pain also slows the drive to breathe — the main way morphine kills in overdose.
- Mu receptors in the gut slow intestinal motion, which is why constipation is almost universal.