Reviewed by MedNext Clinical Team
Codeine is one of the most prescribed opioid analgesics in the world, used for mild-to-moderate pain, cough suppression, and diarrhoea. Yet codeine is a prodrug — it has no intrinsic analgesic activity. Its efficacy depends entirely on hepatic conversion to morphine, a process mediated by the CYP2D6 enzyme. The wide genetic variation in CYP2D6 activity means that the same codeine dose can produce almost no effect in some patients and life-threatening opioid toxicity in others [1].
The CYP2D6 Polymorphism
CYP2D6 is a highly polymorphic enzyme encoded by the CYP2D6 gene on chromosome 22. Over 100 allelic variants have been identified, producing a spectrum of enzyme activity that classifies individuals into four phenotypes [1]:
- Poor metabolisers (PM) — carry two non-functional alleles; produce little or no CYP2D6 activity; convert minimal codeine to morphine; experience little or no analgesia. Approximately 5–10% of Europeans, 1–2% of Asians.
- Intermediate metabolisers (IM) — one reduced-function allele; reduced CYP2D6 activity; variable analgesic response.
- Extensive metabolisers (EM) — two functional alleles; normal CYP2D6 activity; standard analgesic response. The majority of the population.
- Ultra-rapid metabolisers (UM) — carry gene duplications or multiplications (CYP2D6×N); markedly increased enzyme activity; convert codeine to morphine very rapidly, producing supra-therapeutic morphine concentrations. Approximately 1–2% of Northern Europeans, but up to 29% in some North African and Middle Eastern populations.
Ultra-Rapid Metabolisers: Fatal Toxicity
Ultra-rapid metabolisers generate morphine concentrations far exceeding those anticipated from the codeine dose prescribed. This can cause rapid-onset respiratory depression, loss of consciousness, and death — even at doses considered standard and safe for the general population [1]. Several fatal cases have been reported in the literature, including neonatal deaths where breastfed infants were exposed to high morphine concentrations through breast milk of UM mothers taking standard post-partum codeine doses.
This risk is not theoretical. Regulatory agencies in multiple countries have restricted codeine use on this basis. In response to these safety signals, codeine is now contraindicated in children under 12 years of age, in all patients under 18 years following certain surgical procedures (particularly tonsillectomy or adenoidectomy for sleep apnoea), and in breastfeeding women.
Paediatric Contraindication
Children are at higher risk of codeine toxicity for several reasons: CYP2D6 ultra-rapid metaboliser status (the proportion of UMs varies significantly by ethnicity and is unpredictable without genotyping); unpredictable CYP2D6 maturation in young children; and greater sensitivity to opioid-induced respiratory depression. Following the deaths of several children who received codeine following tonsillectomy, regulatory agencies in the UK, EU, and US contraindicated codeine for post-operative analgesia in patients under 18 years, and restricted its use in all children under 12 years [1].
Safer Alternatives
For mild-to-moderate acute pain in children and adults, paracetamol and ibuprofen (used alone or in combination) provide effective analgesia without the unpredictability of CYP2D6-dependent opioid conversion. For moderate-to-severe pain requiring an opioid, drugs that are not prodrugs — morphine, oxycodone, hydromorphone — have more predictable pharmacokinetics, though individual variation in opioid sensitivity remains relevant. Tramadol is also a CYP2D6 substrate for part of its analgesic mechanism and shares some of the same metaboliser-status concerns.
Clinical Pharmacogenomics
The Clinical Pharmacogenetics Implementation Consortium (CPIC) has published guidelines recommending that CYP2D6 ultra-rapid metabolisers avoid codeine entirely, and that poor metabolisers be prescribed an alternative analgesic. Pre-emptive pharmacogenomic testing, where CYP2D6 status is determined once and used to guide all future opioid prescribing, is increasingly available and represents the most robust approach to eliminating codeine-related CYP2D6-driven toxicity [1].
References
- Crews KR, Gaedigk A, Dunnenberger HM, et al. Clinical Pharmacogenetics Implementation Consortium guidelines for cytochrome P450 2D6 genotype and codeine therapy: 2014 update. Clin Pharmacol Ther. 2014;95(4):376-382.