Mechanism of action
Codeine is a prodrug with weak intrinsic opioid activity. Approximately 5-10% of codeine is metabolized by the hepatic cytochrome P450 enzyme CYP2D6 to morphine, which is responsible for most of its analgesic effects. Morphine acts as a selective agonist at mu-opioid receptors in the central nervous system, producing analgesia, euphoria, and respiratory depression.
The antitussive effect of codeine occurs through direct action on the cough center in the medulla, suppressing the cough reflex at doses lower than those required for analgesia. This effect may involve both opioid and non-opioid mechanisms.
CYP2D6 polymorphism significantly affects codeine efficacy and safety:
- Poor metabolizers (5-10% of Caucasians, 1-2% of Asians) produce little to no morphine and experience minimal analgesia
- Ultra-rapid metabolizers (1-2% of Caucasians, up to 29% of certain populations) convert codeine to morphine excessively, risking toxicity and respiratory depression
- Intermediate and extensive metabolizers experience standard therapeutic effects
Other metabolic pathways include glucuronidation to codeine-6-glucuronide and N-demethylation by CYP3A4 to norcodeine, both with minimal analgesic activity.
- Codeine is a prodrug requiring CYP2D6 metabolism to morphine for analgesic effect
- Ultra-rapid CYP2D6 metabolizers risk severe toxicity; poor metabolizers receive minimal analgesia
- Black box warning: contraindicated in children under 12 and post-tonsillectomy patients under 18
- Contraindicated in breastfeeding due to risk of infant respiratory depression
- Most common adverse effect is constipation; most serious is respiratory depression
- Codeine has a ceiling effect for analgesia due to limited conversion capacity
- Schedule II controlled substance with abuse potential and withdrawal risk
- CYP2D6 inhibitors (fluoxetine, paroxetine) reduce efficacy by blocking conversion to morphine
Indications
Codeine is indicated for:
- Mild to moderate pain that is not adequately controlled by non-opioid analgesics
- Nonproductive cough in adults and children over 12 years when other treatments are inadequate
- Combination formulations with acetaminophen or aspirin for enhanced analgesia
Codeine is typically reserved for short-term use due to abuse potential and regulatory concerns. It is no longer recommended in children under 12 years or in adolescents under 18 years following tonsillectomy or adenoidectomy due to risk of severe respiratory depression and death in ultra-rapid metabolizers.
Many guidelines now recommend against codeine as a first-line analgesic due to unpredictable metabolism and availability of safer alternatives such as tramadol or low-dose oxycodone.
Pharmacokinetics
Absorption: Well absorbed from the gastrointestinal tract with peak plasma concentrations reached in 1-2 hours. Oral bioavailability is approximately 50-60% due to first-pass metabolism.
Distribution: Codeine crosses the blood-brain barrier and placenta, and is distributed into breast milk. Volume of distribution is approximately 3-6 L/kg. Plasma protein binding is low (7-25%).
Metabolism: Hepatic metabolism via three pathways:
- CYP2D6 O-demethylation to morphine (5-10%, primary analgesic pathway)
- CYP3A4 N-demethylation to norcodeine (inactive)
- UGT2B7 glucuronidation to codeine-6-glucuronide (inactive)
Elimination: Primarily renal excretion of metabolites and unchanged drug (5-15%). Elimination half-life is 2.5-3.5 hours for codeine and 2-4 hours for morphine. Half-life may be prolonged in renal impairment and elderly patients.
Special populations: Neonates and infants have immature hepatic enzyme systems and are at increased risk of toxicity. Renal impairment requires dose adjustment. Genetic polymorphism in CYP2D6 creates significant inter-individual variability.
Adverse effects
Common adverse effects include:
- Gastrointestinal: Constipation (most common), nausea, vomiting, abdominal pain
- Central nervous system: Drowsiness, sedation, dizziness, headache, confusion
- Respiratory: Respiratory depression, particularly in ultra-rapid CYP2D6 metabolizers
- Dermatologic: Pruritus, urticaria, sweating
- Genitourinary: Urinary retention, difficulty with micturition
Serious adverse effects:
- Respiratory depression is the most serious risk, especially in children, ultra-rapid metabolizers, and when combined with other CNS depressants
- Hypotension and bradycardia
- Seizures at high doses or in predisposed individuals
- Serotonin syndrome when combined with serotonergic drugs
- Adrenal insufficiency with prolonged use
Black Box Warnings:
- Life-threatening respiratory depression and death in children following tonsillectomy/adenoidectomy
- Ultra-rapid metabolism of codeine to morphine
- Risks from concomitant use with benzodiazepines or other CNS depressants
- Risk of medication errors due to confusion between mg and mL
- Neonatal opioid withdrawal syndrome with prolonged maternal use
Dependence and tolerance: Chronic use leads to physical dependence, tolerance, and potential for substance use disorder. Abrupt discontinuation may cause withdrawal symptoms.
Contraindications and warnings
Absolute contraindications:
- Children under 12 years of age
- Postoperative pain management in children under 18 years following tonsillectomy or adenoidectomy
- Known or suspected ultra-rapid CYP2D6 metabolizers
- Significant respiratory depression or acute/severe bronchial asthma in unmonitored settings
- Known or suspected gastrointestinal obstruction, including paralytic ileus
- Hypersensitivity to codeine or other opioids
Relative contraindications and warnings:
- Pregnancy: Codeine crosses the placenta. Prolonged use during pregnancy can cause neonatal opioid withdrawal syndrome. Use only if benefit outweighs risk. Category C in older classification systems.
- Breastfeeding: Codeine and morphine are excreted in breast milk. Ultra-rapid metabolizers may produce high morphine levels in milk, causing infant sedation and respiratory depression. Use is contraindicated in nursing mothers.
- Renal impairment: Metabolites accumulate; dose reduction required
- Hepatic impairment: Increased risk of adverse effects due to altered metabolism
- Elderly patients: Increased sensitivity to CNS and respiratory effects
- Head injury or increased intracranial pressure: May obscure clinical course and increase CO₂ retention
- Concurrent CNS depressants: Risk of profound sedation and respiratory depression
- Acute alcoholism or delirium tremens
- Adrenal insufficiency, hypothyroidism, prostatic hypertrophy
Abuse potential: Codeine is a Schedule II controlled substance in the United States (Schedule III when combined with acetaminophen or aspirin below certain thresholds). It has abuse potential and risk for diversion.
Drug interactions
CNS depressants (benzodiazepines, barbiturates, alcohol, sedating antihistamines, other opioids):
- Additive respiratory depression, sedation, hypotension, and risk of death
- Avoid combination or use lowest effective doses with close monitoring
CYP2D6 inhibitors (fluoxetine, paroxetine, quinidine, bupropion):
- Reduce conversion to morphine, decreasing analgesic efficacy
- May need alternative analgesic
CYP3A4 inducers (rifampin, carbamazepine, phenytoin):
- Shunt codeine metabolism toward norcodeine formation via CYP3A4, thereby reducing conversion to morphine and decreasing analgesic efficacy
- May require alternative analgesic or dose adjustment
CYP3A4 inhibitors (ketoconazole, erythromycin, grapefruit juice):
- May increase codeine levels
Serotonergic drugs (SSRIs, SNRIs, MAOIs, triptans, linezolid):
- Risk of serotonin syndrome with symptoms including agitation, hyperthermia, hyperreflexia, clonus
- Avoid MAOIs within 14 days of codeine
Anticholinergic drugs:
- Increased risk of urinary retention and severe constipation, potentially leading to paralytic ileus
Muscle relaxants:
- Enhanced neuromuscular blocking effects and respiratory depression
Diuretics:
- Opioids may reduce efficacy of diuretics in heart failure patients
Mixed agonist/antagonist opioids (nalbuphine, butorphanol, pentazocine):
- May reduce analgesic effect of codeine or precipitate withdrawal
Clinical pearls
- CYP2D6 testing should be considered before prescribing codeine in populations with high prevalence of ultra-rapid or poor metabolizer phenotypes, or in patients with unexpected response.
- Equianalgesic dosing: Oral codeine is approximately one-seventh as potent as oral morphine, reflecting codeine's status as a weak opioid requiring conversion to morphine.
- Constipation prophylaxis with stimulant laxatives should be initiated at the start of therapy, as tolerance to this adverse effect does not develop.
- Duration of therapy should be limited to the shortest duration necessary. Many guidelines recommend 3-7 days maximum for acute pain.
- Ceiling effect: Codeine has a ceiling effect for analgesia due to limited conversion capacity to morphine. Increasing doses beyond a certain threshold increases adverse effects without improving analgesia.
- Antitussive use: For cough suppression, lower doses are effective compared to analgesic doses.
- Tramadol as alternative: Like codeine, tramadol is also a prodrug requiring CYP2D6 metabolism, but it has additional mechanisms (norepinephrine and serotonin reuptake inhibition) that may provide benefit in CYP2D6 poor metabolizers.
- Naloxone reversal: Codeine overdose and respiratory depression can be reversed with naloxone, though higher or repeated doses may be needed compared to direct-acting opioids.
- Withdrawal management: If physical dependence has developed, taper gradually by 10-25% every few days to minimize withdrawal symptoms.
- Exam tip: Questions involving unexpected lack of analgesic response to codeine or severe toxicity at standard doses should trigger consideration of CYP2D6 polymorphism.
Practice questions
1. A 7-year-old girl is brought to the emergency department 6 hours after undergoing adenotonsillectomy. Her mother reports that she has been increasingly difficult to arouse and is breathing slowly. Vital signs show temperature 37.2°C, pulse 65/min, respirations 8/min, and blood pressure 95/60 mm Hg. Physical examination shows miotic pupils and decreased responsiveness to painful stimuli. She received standard postoperative doses of codeine for pain control. Genetic testing reveals she is a CYP2D6 ultra-rapid metabolizer. Which of the following best explains this patient's condition?
2. A 45-year-old man with chronic lower back pain reports that codeine, which previously provided adequate pain relief, is no longer effective. He has been taking codeine for 6 months and has recently started fluoxetine for depression. He denies taking extra doses or misusing his medication. His pain intensity has increased significantly over the past 2 weeks. Physical examination is unchanged from previous visits. Which of the following best explains the loss of analgesic efficacy?
3. A 32-year-old woman is prescribed codeine for pain following a dental extraction. After taking two doses, she experiences no pain relief and rates her pain as severe. She has no history of opioid use or substance abuse. Her past medical history is unremarkable except for lack of response to tramadol prescribed after a previous procedure. Which of the following is the most likely explanation for her lack of response to codeine?
Frequently asked questions
How does codeine work?
Codeine is a prodrug that works by being converted in the liver by the CYP2D6 enzyme into morphine, which then acts on mu-opioid receptors in the brain and spinal cord to reduce pain perception. Approximately 5-10% of codeine is converted to morphine, which is responsible for most of its pain-relieving effects. The effectiveness of codeine varies significantly between individuals based on their CYP2D6 genetic makeup.
What are the side effects of codeine?
Common side effects include constipation (the most frequent), nausea, vomiting, drowsiness, dizziness, and itching. Serious side effects include respiratory depression (slowed or difficult breathing), which can be life-threatening, especially in children and ultra-rapid CYP2D6 metabolizers. Other serious effects include severe allergic reactions, confusion, low blood pressure, and urinary retention. With long-term use, physical dependence and addiction can develop.
Is codeine a controlled substance?
Yes, codeine is a Schedule II controlled substance in the United States when prescribed alone. When combined with other medications like acetaminophen or aspirin below certain concentration thresholds, it may be classified as Schedule III. This classification reflects its medical usefulness but also its potential for abuse, dependence, and diversion. Prescriptions for codeine are subject to specific regulations and monitoring.
Can you take codeine while pregnant or breastfeeding?
Codeine should generally be avoided during pregnancy, especially for prolonged periods, as it crosses the placenta and can cause neonatal opioid withdrawal syndrome in newborns. It is contraindicated during breastfeeding because codeine and its active metabolite morphine pass into breast milk and can cause serious sedation and breathing problems in nursing infants, particularly if the mother is an ultra-rapid CYP2D6 metabolizer.
Codeine vs tramadol — what is the difference?
Both codeine and tramadol are weak opioids used for mild to moderate pain, and both are prodrugs requiring CYP2D6 metabolism for full effect. However, tramadol has additional mechanisms of action including inhibition of norepinephrine and serotonin reuptake, which may provide some analgesic benefit even in poor CYP2D6 metabolizers. Tramadol generally has fewer respiratory depression concerns than codeine but carries a risk of serotonin syndrome when combined with other serotonergic drugs. Both are controlled substances with abuse potential.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: R05DA04
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.