Codeine: Why the Same Dose Affects Everyone Differently

Codeine, one of the most commonly used opioid painkillers in over-the-counter medications, affects individuals in dramatically different ways depending on how their bodies process the drug. This variability stems from genetic differences in liver enzymes responsible for converting codeine into its active form, morphine, leading to outcomes ranging from ineffective pain relief to dangerous overdose risks even at standard doses.

The key factor behind these differences is the CYP2D6 enzyme, which metabolizes codeine into morphine. According to medical research, people vary widely in how efficiently this enzyme works. Most individuals are normal metabolizers, producing enough morphine to relieve mild to moderate pain. However, a significant portion of the population falls into categories that either process the drug too slowly or too quickly, leading to either lack of effect or heightened risk.

Approximately 2% to 11% of people are intermediate metabolizers, meaning they convert codeine to morphine at a slower-than-average rate. For these individuals, standard doses may provide little to no pain relief because insufficient morphine is produced. At the other end of the spectrum, ultra-rapid metabolizers — estimated to make up 1% to 2% of the population — convert codeine into morphine much more quickly and completely. This rapid conversion can result in unexpectedly high levels of morphine in the bloodstream, increasing the risk of severe side effects such as extreme drowsiness, slowed or difficult breathing, and in rare cases, life-threatening respiratory depression.

These genetic variations are not evenly distributed across populations. Ultra-rapid metabolism is more commonly found in individuals of North African and Middle Eastern descent, while poor metabolism — where the body has little to no ability to convert codeine to morphine — is more prevalent among certain Caucasian groups. This means that two people taking the same dose of codeine-containing medication, such as co-codamol or Nurofen Plus, could have vastly different experiences: one may feel no pain relief, while another may suffer from opioid toxicity.

Because of this unpredictability, health authorities in several countries have placed restrictions on codeine use. In Australia, for example, codeine is no longer available over the counter and requires a prescription. Medical guidelines now recommend avoiding codeine in certain populations, including children under 12, breastfeeding mothers, and individuals known to be ultra-rapid metabolizers, due to the risk of morphine passing into breast fat or causing respiratory depression in infants. The drug is also generally not recommended for long-term or chronic pain management, as safer and more predictable alternatives are available.

Patients are advised to be aware of how they personally respond to codeine and to consult a healthcare provider if they experience unusual drowsiness, difficulty breathing, or lack of pain relief after taking the medication. Pharmacogenomic testing, which analyzes genes like CYP2D6, is available in some clinical settings and can help predict how an individual will metabolize certain opioids, although it is not yet routine practice.

As opioid safety remains a public health priority, understanding the role of genetics in drug response underscores the importance of personalized approaches to medication. What works safely for one person may pose serious risks to another — a reality that challenges the one-size-fits-all model of dosing and highlights the need for greater awareness of how individual biology shapes treatment outcomes.