Raw research findings for this medication. Mostly single studies, not curated or confirmed robust.
Codeine is an opioid used for mild to moderate pain and, in some countries, for dry cough. It is often combined with paracetamol (acetaminophen) in one tablet. On its own it has little effect: most of its pain relief comes from the morphine the body makes from it.
The liver enzyme CYP2D6 turns codeine into morphine. People with two non-working CYP2D6 copies (poor metabolizers, about 5 to 10 in 100 people of European ancestry) make very little morphine and get little pain relief. People with extra working copies (ultrarapid metabolizers, common in parts of North Africa, the Middle East and Ethiopia) make more morphine faster, with a risk of serious opioid side effects. CPIC and DPWG have published guidelines on CYP2D6 and codeine, and the US drug label warns about ultrarapid metabolizers. A few other genes have been studied, with weaker evidence.
Clinical guidelines that name a gene for Codeine. They sort people into groups by the gene copies they carry.
CPIC guideline: CYP2D6. CYP2D6 activity score 0 to 6 or more. Strong. 2021.
DPWG guideline: CYP2D6. CYP2D6 activity score 0 to 6 or more. 2022.
US FDA: CYP2D6. CYP2D6 activity score 2.5 to 6 or more.
Every published finding on gene variants and Codeine. Upload your DNA file to see which of these rows apply to you.
PharmGKB weighs all studies of a variant and rates how strong the evidence is, from 1A (strongest) to 4.
CYP2D6 CYP2D6*1, CYP2D6*1xN, CYP2D6*2, CYP2D6*2xN, CYP2D6*3, CYP2D6*4, CYP2D6*5, CYP2D6*6, CYP2D6*10, CYP2D6*17, CYP2D6*29, CYP2D6*36, CYP2D6*41: Level 1A · replicated. Metabolism/PK. 19.
CYP2D6 CYP2D6*1, CYP2D6*1xN, CYP2D6*2, CYP2D6*2xN: Level 1A · replicated. Toxicity: Pain. 8.
CYP2D6 CYP2D6*1, CYP2D6*4, CYP2D6*5, CYP2D6*6, CYP2D6*17, CYP2D6*40: Level 1A · replicated. Efficacy: Pain. 7.
CYP2D6 CYP2D6*1, CYP2D6*24: Level 3. Metabolism/PK. 2.
Studies of how quickly the body processes this medication.
Studies of how well it worked in people carrying a variant.
Studies of how much of the drug people needed or tolerated.
Study findings and evidence levels: PharmGKB (CC BY-SA 4.0). Guidelines: CPIC and DPWG, as bundled in PharmCAT. Most studies were done mainly in people of European ancestry.
More drugs from the same category
Not a prescribing recommendation. This page is educational information about pharmacogenomic research and clinical guidelines - it is not medical or dosing advice, and not a prediction of how any medication will work for you. Never start, stop, or change a medication or dose based on it; talk to your doctor or pharmacist, who can order clinical-grade testing if appropriate.
Content updated September 2026.
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