Nairobi, April 28 -- 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 analgesi...