morphine

Morphine is the original strong painkiller — the molecule against which all other opioids are measured. Isolated from opium in the early 1800s and named after Morpheus, the Greek god of dreams, it is the reference standard so universal that the strength of other opioids is expressed as a ratio to it (morphine-equivalent dose).

Morphine is a mu opioid receptor agonist. By switching on mu receptors in the brain and spinal cord, it powerfully relieves severe pain, calms the cough reflex, and eases the breathlessness of dying patients. Its predictable burdens come with it: nausea, constipation, sedation, pupil constriction, and — at high doses — dangerous slowing of breathing. It is a controlled drug because the same receptor that relieves pain also produces euphoria, tolerance, and dependence.

Pharmacologically it is a useful teaching example because it behaves badly in textbook ways. It is poorly fat-soluble and not very well absorbed by mouth, undergoing heavy first-pass metabolism, so oral doses must be much larger than intravenous ones. It is conjugated in the liver to morphine-6-glucuronide, an active metabolite that itself relieves pain and accumulates dangerously in kidney failure — a reminder that a drug's metabolites can matter as much as the parent drug.