Codeine conversion and pharmacology
A weak opioid used for mild to moderate pain and as an antitussive. A prodrug of morphine.
Equianalgesic estimate
200mg Oral ≈ 30mg Oral Morphine (Highly variable)
Published equianalgesic ratios vary between references and with clinical context. Read how to interpret conversion results before applying an estimate.
Mechanism of action
Weak mu-opioid receptor agonist. Antitussive effect via direct action on the medulla.
Brand names: Tylenol #3 (w/ APAP), Tylenol #4 (w/ APAP)
Pharmacokinetics
- Protein binding
- 7-25%
- Volume of distribution
- 3-6 L/kg
- Half-life
- 2.5-4 hours
| Route | Form | Bioavailability | Onset | Peak | Duration |
|---|---|---|---|---|---|
| Oral | — | ~90% | 30-60 min | 60 min | 4-6 hours |
Metabolism
- Glucuronidation (UGT2B7) (major)
- CYP2D6 (O-demethylation)
- CYP3A4 (N-demethylation)
Metabolites
- Morphine Responsible for primary analgesic effect (formed via CYP2D6).
- Codeine-6-glucuronide (C6G) Potential analgesic contribution.
Prodrug. Efficacy and toxicity are highly dependent on CYP2D6 genotype. CYP2D6 inhibitors (e.g., fluoxetine, paroxetine, bupropion) significantly reduce efficacy.
Renal and hepatic impairment
Renal
Avoid. Metabolites (including morphine metabolites M3G/M6G) accumulate, leading to high risk of toxicity (narcosis, respiratory depression).
Hepatic
Conversion to morphine may be reduced.
Clinical cautions
- CYP2D6 Poor Metabolizers (approx. 7-10% of Caucasians) experience little to no analgesia.
- CYP2D6 Ultrarapid Metabolizers are at risk of fatal morphine overdose.
- FDA Black Box Warning: Contraindicated in children <12 years and post-tonsillectomy/adenoidectomy.
For qualified healthcare professionals. This page is an educational reference, not prescribing advice. Check medicine information and conversion factors against a current clinical source and local guidance.
Other opioids in this reference
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Last updated 30 August 2026. Read how to interpret conversion results and the Terms of Service.