For a medicine so commonly found in bathroom cabinets and high street pharmacies, codeine has a surprisingly complicated story. It sits at the intersection of pain relief, genetics, public health and regulation. As the UK continues to tighten rules around opioid use, codeine offers a useful case study in how a drug can be both helpful and potentially harmful, depending on who takes it and how it is used.

Codeine is an opioid used to treat mild to moderate pain. In some formulations, it is also used to suppress coughing. Over-the-counter products typically combine it with paracetamol, as in co-codamol, or ibuprofen, while stronger doses are available only on prescription.

Codeine itself is a weak opioid. Its analgesic effect is about one tenth that of morphine. Once swallowed, it is metabolised by enzymes in the liver, with some of it converted into morphine. That morphine then produces pain relief by acting on opioid receptors in the brain. For most people, the body makes enough morphine to ease symptoms. For others, the same dose can be ineffective or unexpectedly strong.

One of the most striking features of codeine is how differently people process it. The enzyme mainly responsible for converting codeine into morphine, CYP2D6, varies significantly between people. Most metabolise codeine at an expected rate, but some carry genetic variants that alter the process.

A small proportion of the........

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