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Antitussive (cough suppressant)

Dextromethorphan

Dextromethorphan is a centrally acting antitussive agent available over-the-counter for symptomatic relief of nonproductive cough. It acts primarily as an NMDA receptor antagonist in the cough center of the medulla. The most testable fact is its potential for serotonin syndrome when combined with MAO inhibitors or SSRIs, and its abuse potential at supratherapeutic doses.

Practice questionsHigh-yield facts
Drug classAntitussive, NMDA receptor antagonist
MechanismNMDA receptor antagonist in medullary cough center
Main usesSymptomatic relief of nonproductive cough
Key adverse effectSerotonin syndrome with serotonergic drugs, dissociation at high doses
5 min readFor medical studentsUpdated 2026-10-15

Mechanism of action

Dextromethorphan suppresses cough through multiple mechanisms. It is a noncompetitive antagonist at N-methyl-D-aspartate (NMDA) receptors in the cough center located in the medulla oblongata. By blocking these glutamate receptors, it raises the threshold for cough reflex activation.

Additionally, dextromethorphan acts as a sigma-1 receptor agonist, which contributes to its antitussive effects and may provide neuroprotective properties. Unlike codeine and other opioid antitussives, dextromethorphan does not bind to opioid receptors at therapeutic doses and lacks analgesic or addictive properties in normal use.

Dextromethorphan is the D-isomer of the codeine analog levorphanol but lacks opioid activity. It is metabolized primarily by CYP2D6 to dextrorphan, an active metabolite that also has NMDA antagonist activity. In poor CYP2D6 metabolizers or when CYP2D6 is inhibited, dextromethorphan levels increase, potentially leading to enhanced effects or toxicity.

High-yield for exams
  • Dextromethorphan is an NMDA receptor antagonist in the medullary cough center, not an opioid receptor agonist
  • Contraindicated with MAO inhibitors; combination causes serotonin syndrome and hypertensive crisis
  • CYP2D6 metabolizes dextromethorphan to active metabolite dextrorphan; poor metabolizers have increased toxicity risk
  • At supratherapeutic doses (abuse), produces PCP-like dissociative and hallucinogenic effects
  • Serotonin syndrome risk when combined with SSRIs, SNRIs, or other serotonergic drugs
  • Quinidine (CYP2D6 inhibitor) dramatically increases dextromethorphan levels up to 20-fold
  • Not recommended for children under 4 years; no proven efficacy and potential for serious adverse effects
  • Should not suppress productive cough as it impairs mucus clearance and may worsen respiratory infections

Indications

Dextromethorphan is indicated for:

  • Symptomatic relief of nonproductive (dry) cough associated with upper respiratory tract infections, common cold, bronchitis, and other conditions causing cough without significant sputum production

It is available over-the-counter in various formulations including syrups, lozenges, and extended-release suspensions. Dextromethorphan is often combined with other agents such as guaifenesin (expectorant), antihistamines, or decongestants in multi-symptom cold preparations.

Dextromethorphan is not effective for cough due to smoking, asthma, emphysema, or chronic bronchitis. It should not be used to suppress productive cough, as clearing secretions is important in these conditions.

Pharmacokinetics

Absorption: Dextromethorphan is rapidly absorbed from the gastrointestinal tract following oral administration. Peak plasma concentrations occur within 2-3 hours.

Distribution: It is widely distributed throughout the body and crosses the blood-brain barrier to reach its site of action in the medulla.

Metabolism: Dextromethorphan undergoes extensive first-pass metabolism in the liver, primarily via CYP2D6 to dextrorphan (active metabolite) and via CYP3A4 to 3-methoxymorphinan (inactive). Approximately 5-10% of the population are poor CYP2D6 metabolizers, which can lead to higher plasma levels of the parent drug and altered effects. In extensive metabolizers, dextrorphan contributes significantly to the clinical effect.

Elimination: Metabolites are excreted primarily in urine. The half-life is approximately 2-4 hours in extensive metabolizers but can be prolonged to 24 hours or more in poor metabolizers.

Onset and duration: Antitussive effects begin within 15-30 minutes and last 4-6 hours for immediate-release formulations.

Adverse effects

At therapeutic doses, dextromethorphan is generally well tolerated. Common adverse effects include:

  • Dizziness
  • Drowsiness
  • Nausea
  • Gastrointestinal upset

Serious adverse effects

Serotonin syndrome is the most clinically significant risk, particularly when dextromethorphan is combined with other serotonergic agents such as SSRIs, SNRIs, MAO inhibitors, or linezolid. Symptoms include agitation, hyperthermia, hyperreflexia, clonus, tremor, diaphoresis, and in severe cases, seizures and death.

Abuse and toxicity: At supratherapeutic doses (10-50 times therapeutic doses), dextromethorphan produces dissociative effects similar to ketamine and phencyclidine (PCP), leading to abuse potential particularly among adolescents. High-dose effects include:

  • Euphoria and dissociation
  • Hallucinations
  • Confusion and agitation
  • Ataxia and slurred speech
  • Tachycardia and hypertension
  • Respiratory depression (rare)
  • Hyperthermia

Central nervous system depression can occur, especially when combined with alcohol or other CNS depressants.

Allergic reactions including rash and pruritus are uncommon but reported.

Contraindications and warnings

Absolute contraindications

  • MAO inhibitor use within the past 14 days (risk of serotonin syndrome, hypertensive crisis)
  • Hypersensitivity to dextromethorphan or formulation components

Warnings and precautions

  • Serotonergic drugs: Use with extreme caution or avoid in patients taking SSRIs, SNRIs, tricyclic antidepressants, triptans, or other serotonergic agents due to serotonin syndrome risk
  • CYP2D6 inhibitors: Drugs that inhibit CYP2D6 (e.g., fluoxetine, paroxetine, quinidine, bupropion) can increase dextromethorphan levels and risk of toxicity
  • Chronic or persistent cough: Should not be used without medical evaluation, as it may mask serious underlying conditions such as asthma, emphysema, or heart failure
  • Productive cough: Contraindicated when cough is producing significant sputum, as suppression may impair clearance of secretions
  • Hepatic impairment: Use with caution as metabolism may be impaired
  • Pregnancy: Category C. Limited human data; use only if benefit outweighs risk
  • Pediatric use: Not recommended for children under 4 years due to potential for serious adverse effects and lack of proven efficacy in this age group

Abuse potential: Healthcare providers should be aware of the potential for recreational abuse, particularly in adolescent populations.

Drug interactions

Major interactions

MAO inhibitors (phenelzine, tranylcypromine, selegiline): Contraindicated combination. Risk of serotonin syndrome, hyperpyrexia, hypertension, and death. Must allow 14-day washout period.

Serotonergic drugs (SSRIs, SNRIs, TCAs, triptans, linezolid, tramadol, St. John's wort): Increased risk of serotonin syndrome. Monitor closely or avoid combination if possible.

CYP2D6 inhibitors (quinidine, fluoxetine, paroxetine, bupropion): Increase dextromethorphan plasma levels, potentially causing toxicity. Quinidine can increase levels up to 20-fold. Reduce dextromethorphan dose or avoid combination.

Moderate interactions

CNS depressants (alcohol, benzodiazepines, opioids, sedating antihistamines): Additive sedation and CNS depression. Advise patients to avoid alcohol and use caution with other sedating medications.

CYP3A4 inhibitors (ketoconazole, erythromycin, grapefruit juice): May moderately increase dextromethorphan levels through secondary metabolic pathway.

Amiodarone: Has been reported to increase dextromethorphan levels through CYP2D6 and CYP3A4 inhibition.

Clinical note

Many over-the-counter combination products contain dextromethorphan along with other active ingredients (acetaminophen, antihistamines, decongestants). Review all medications to avoid unintentional drug interactions and duplication of ingredients.

Clinical pearls

  • NMDA antagonism at the cough center is the primary mechanism; dextromethorphan lacks opioid receptor activity at therapeutic doses
  • Not a controlled substance in most jurisdictions, but some states and countries have implemented purchase restrictions due to abuse potential
  • CYP2D6 polymorphism affects clinical response: poor metabolizers may experience prolonged effects or toxicity at standard doses
  • Serotonin syndrome is the most tested adverse effect—always consider drug interactions with serotonergic agents
  • High-dose abuse produces dissociative, hallucinogenic effects similar to ketamine or PCP; termed "robotripping" in street vernacular
  • Ineffective for chronic cough: Should prompt evaluation for underlying causes such as GERD, postnasal drip, asthma, or ACE inhibitor use
  • The "DXM plateau" phenomenon describes dose-dependent effects from mild stimulation to complete dissociation at abuse doses
  • Quinidine + dextromethorphan combination (marketed as Nuedexta) is FDA-approved for pseudobulbar affect; quinidine inhibits CYP2D6 to increase dextromethorphan levels
  • Formulation matters: Extended-release products should not be crushed or chewed
  • Pediatric safety: FDA and professional organizations recommend against use in children under 4 years; use caution in children 4-6 years
  • Consider dextromethorphan toxicity in the differential for altered mental status, especially in adolescents presenting with confusion, hallucinations, or agitation
  • Not recommended for suppressing productive cough as it may lead to mucus retention and secondary infection

Practice questions

1. A 16-year-old male is brought to the emergency department by friends after being found confused and agitated at a party. He has horizontal nystagmus, ataxia, slurred speech, and is experiencing visual hallucinations. His heart rate is 118/min and blood pressure is 148/92 mmHg. His friends report he consumed multiple bottles of cough syrup. Urine drug screen is negative for opioids, amphetamines, and cocaine. Which of the following is the most likely mechanism of action of the substance ingested?

Answer: B. This patient presents with classic signs of dextromethorphan toxicity at supratherapeutic doses: dissociation, nystagmus, ataxia, hallucinations, and sympathomimetic effects. Dextromethorphan is an NMDA receptor antagonist that produces PCP-like dissociative effects at high doses. The negative opioid screen rules out codeine-containing cough syrups. Mu-opioid agonists would cause miosis and respiratory depression. GABA-A agonists cause sedation without hallucinations or nystagmus.

2. A 45-year-old woman with depression treated with fluoxetine for the past 3 months develops a nonproductive cough from an upper respiratory infection. She takes over-the-counter dextromethorphan as directed. Two days later, she presents to the emergency department with agitation, diaphoresis, tremor, hyperreflexia, and a temperature of 39.2°C (102.6°F). Which of the following best explains this patient's presentation?

Answer: B. This patient has serotonin syndrome resulting from the combination of fluoxetine (SSRI) and dextromethorphan, both serotonergic agents. Classic features include hyperthermia, agitation, hyperreflexia, tremor, and diaphoresis. Fluoxetine also inhibits CYP2D6, increasing dextromethorphan levels. Neuroleptic malignant syndrome presents with rigidity rather than hyperreflexia and requires antipsychotic exposure. Anticholinergic toxicity causes dry skin (not diaphoresis) and hyperthermia with delirium.

3. A 28-year-old man is started on dextromethorphan for persistent dry cough. He is a known CYP2D6 poor metabolizer. Compared to extensive metabolizers taking the same dose, which of the following is most likely in this patient?

Answer: C. CYP2D6 poor metabolizers cannot efficiently convert dextromethorphan to its active metabolite dextrorphan, leading to accumulation of the parent drug. This results in higher plasma concentrations and prolonged half-life, increasing the risk of adverse effects including CNS toxicity. Poor metabolizers actually have prolonged (not shorter) duration of effect. While some antitussive activity may be reduced due to less dextrorphan formation, the parent compound still has NMDA antagonist activity.

Frequently asked questions

How does dextromethorphan work?

Dextromethorphan works by blocking NMDA receptors in the cough center of the medulla oblongata, raising the threshold for cough reflex activation. It also acts as a sigma-1 receptor agonist. Unlike codeine, it does not work through opioid receptors at therapeutic doses and therefore lacks analgesic properties and typical opioid side effects like respiratory depression or physical dependence at recommended doses.

What are the side effects of dextromethorphan?

Common side effects include dizziness, drowsiness, nausea, and gastrointestinal upset. The most serious adverse effect is serotonin syndrome when combined with antidepressants or MAO inhibitors, causing agitation, hyperthermia, and potentially seizures. At high doses (abuse), dextromethorphan causes dissociation, hallucinations, confusion, tachycardia, and hyperthermia similar to PCP or ketamine intoxication.

Is dextromethorphan a controlled substance?

Dextromethorphan is not a federally controlled substance in the United States, though some states have restricted sales to minors due to abuse potential. Unlike codeine-containing cough suppressants, dextromethorphan does not act on opioid receptors and is available over-the-counter. However, it has significant abuse potential at supratherapeutic doses, producing dissociative effects, and is monitored by poison control centers for toxicity cases.

Can you take dextromethorphan with antidepressants?

Combining dextromethorphan with antidepressants, especially SSRIs, SNRIs, or MAO inhibitors, carries significant risk of serotonin syndrome. MAO inhibitors are absolutely contraindicated within 14 days of dextromethorphan use. With SSRIs and SNRIs, the risk is lower but still present, particularly at higher doses. Always consult a healthcare provider before combining these medications, and be vigilant for symptoms of serotonin syndrome.

Dextromethorphan vs codeine — what is the difference?

Dextromethorphan and codeine are both antitussives but work differently. Dextromethorphan blocks NMDA receptors and has no opioid activity at therapeutic doses, while codeine is an opioid that suppresses cough through mu-opioid receptor activation. Codeine is a controlled substance with addiction potential, causes constipation and respiratory depression, and has analgesic properties. Dextromethorphan is available OTC, lacks typical opioid effects, but has abuse potential for dissociative effects at high doses.

Conditions it is used for

Nonproductive coughDry coughCough from upper respiratory infectionCough from common coldCough from acute bronchitis
CodeineBenzonatateGuaifenesinDiphenhydraminePromethazineHydrocodoneKetamineMemantine

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: R05DA09

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.