Codeine and CYP2D6 Pharmacogenomics
Last updated: February 2026
What Is Codeine?
Codeine is an opioid analgesic used to treat mild to moderate pain and, in some formulations, to suppress cough. It is one of the most widely prescribed opioids globally and is available in many countries both by prescription and in low-dose combination products.
What makes codeine uniquely important in pharmacogenomics is that it is a prodrug. Codeine itself has relatively weak opioid activity. To produce its analgesic effect, it must be converted to morphine in the liver by the enzyme the CYP2D6 enzyme. This means the pain-relieving effectiveness of codeine depends almost entirely on your CYP2D6 metabolizer status — making the codeine–CYP2D6 interaction one of the most clinically significant drug-gene pairs.
How CYP2D6 Converts Codeine to Morphine
After ingestion, codeine undergoes several metabolic pathways. The most clinically important is O-demethylation by CYP2D6, which converts codeine to morphine. Morphine is responsible for the majority of codeine's analgesic effect and also its opioid-related side effects.
Approximately 5–10% of codeine is converted to morphine via this pathway. Although this seems like a small fraction, morphine is far more potent than codeine — approximately 10 times more potent as an opioid receptor agonist. Even small variations in morphine production can have clinically meaningful effects on both pain relief and safety. Learn more about how pharmacogenomic testing works from raw DNA data.
Have 23andMe or AncestryDNA raw data? Find out if codeine is flagged for your CYP2D6 genotype.
Impact of CYP2D6 Metabolizer Status
Your CYP2D6 phenotype directly determines how much morphine your body produces from codeine:
- Ultrarapid Metabolizer (UM): Increased CYP2D6 activity leads to faster and greater conversion of codeine to morphine. This can result in dangerously elevated morphine levels, potentially causing respiratory depression, excessive sedation, and in severe cases, death. CPIC guidelines recommend avoiding codeine entirely.
- Normal Metabolizer (NM): Standard CYP2D6 function. Codeine is converted to morphine at the intended rate. Standard dosing applies.
- Intermediate Metabolizer (IM): Reduced CYP2D6 activity. Lower morphine production may lead to reduced analgesic effectiveness. Some patients may require alternative pain management.
- Poor Metabolizer (PM): Severely reduced or absent CYP2D6 activity. Very little or no morphine is produced, resulting in minimal analgesic benefit. CPIC guidelines recommend avoiding codeine and using an alternative analgesic.
For a plain-language explanation of these metabolizer categories and how they affect drug response, see what poor metabolizer status means.
FDA Boxed Warning
In 2017, the FDA issued a strengthened boxed warning — its most serious safety communication — restricting codeine use in children under 12 and in adolescents aged 12–18 following tonsillectomy or adenoidectomy. The FDA also warned against codeine use by breastfeeding mothers, as morphine can pass into breast milk at dangerous concentrations in ultrarapid metabolizers.
These warnings followed reports of serious adverse events, including deaths, in children who were CYP2D6 ultrarapid metabolizers. The cases demonstrated how CYP2D6 genetic variation can transform a commonly prescribed medication into a serious safety risk in specific populations.
Already have your DNA file? Check whether your CYP2D6 status affects codeine safety.
CPIC Guideline Summary
The codeine–CYP2D6 interaction has a CPIC Level A classification — the strongest evidence level. The CPIC guideline (Crews et al., 2014; PMID: 24458010) provides specific recommendations:
- NM: Standard codeine dosing. Monitor for efficacy and adverse effects as expected.
- IM: Codeine may provide reduced analgesia. If adequate pain relief is not achieved, consider an alternative analgesic not metabolized by CYP2D6.
- PM: Avoid codeine. Use an alternative analgesic such as a non-opioid or an opioid not dependent on CYP2D6 (e.g., morphine, oxycodone). Tramadol and hydrocodone share CYP2D6 dependence and present similar concerns.
- UM: Avoid codeine due to the risk of toxicity from rapid morphine formation. Use an alternative analgesic regardless of clinical indication.
Understanding Your Results
If you have raw DNA data from 23andMe, AncestryDNA, or another consumer service, DecodeMyBio can analyze your CYP2D6 status and report whether codeine is flagged for your genotype. Your Medication Safety Report will include your CYP2D6 diplotype, activity score, metabolizer phenotype, and the specific CPIC recommendation for codeine.
Because codeine safety depends directly on CYP2D6 status, this is one of the most clinically actionable findings in a pharmacogenomic report. If you are identified as a CYP2D6 ultrarapid or poor metabolizer, this information is particularly relevant to share with your provider. See our methodology for how results are derived, our limitations page for important caveats, and our guide to understanding your report. To see how DecodeMyBio compares with other pharmacogenomic services, visit our pharmacogenomic testing comparison.
Get your Medication Safety Report. Upload your raw DNA data to see your CYP2D6 results and codeine safety profile.
Related Resources
Frequently Asked Questions
Why does codeine require CYP2D6 to work?
Codeine is a prodrug with weak opioid activity on its own. It must be converted to morphine by CYP2D6 in the liver to produce its pain-relieving effect. Without adequate CYP2D6 function, very little morphine is produced and the drug is largely ineffective.
What happens if a CYP2D6 ultrarapid metabolizer takes codeine?
Ultrarapid metabolizers convert codeine to morphine faster and in greater quantities than normal. This can lead to dangerously elevated morphine levels, causing respiratory depression, excessive sedation, and potentially life-threatening toxicity. CPIC guidelines recommend avoiding codeine entirely.
Can you take codeine safely if you are a CYP2D6 poor metabolizer?
Poor metabolizers produce very little morphine from codeine, so the drug provides minimal pain relief. The lack of efficacy makes codeine an inappropriate choice. CPIC guidelines recommend using an alternative analgesic.
Does the FDA codeine warning apply to all opioids?
No. The FDA boxed warning is specific to codeine and tramadol, both of which require CYP2D6 for activation. Other opioids such as morphine and oxycodone do not depend on CYP2D6 for their analgesic effect.
Can consumer DNA tests detect CYP2D6 variants relevant to codeine?
Yes. Consumer arrays from 23andMe and AncestryDNA include many CYP2D6 star allele variants. However, gene deletions and duplications — important for poor and ultrarapid metabolizer classification — cannot be reliably detected from array data.
What are alternatives to codeine for patients with CYP2D6 variants?
CPIC guidelines suggest non-opioid analgesics or opioids not dependent on CYP2D6, such as morphine or oxycodone at appropriate doses. Tramadol and hydrocodone also require CYP2D6 and present similar concerns.
Last reviewed: February 2026 · DecodeMyBio Editorial Team