dextromethorphan/quinidine

Action and Spectrum

Actions and Spectrum: 

dextromethorphan/quinidine is a combination medication with two active ingredients: dextromethorphan and quinidine. This combination is used to treat pseudobulbar affect (PBA), a neurological disorder characterized by uncontrollable episodes of crying or laughing that are out of proportion to the individual’s emotional state.

quinidine is included in the formulation to inhibit the breakdown of dextromethorphan in the body, leading to increased dextromethorphan levels and enhanced therapeutic effects. 

Drug Interaction

Adverse Reaction

Black Box Warning

Black Box Warning: 

None 

Contraindication / Caution

Contraindication/Caution: 

Contraindications: 

• Hypersensitivity 
• Concurrent use of MAO inhibitors 
• History of thrombocytopenia 

Cautions: 

• Cardiac issues 
• Liver impairment 
• Renal impairment 
• Respiratory issues 
• Drug interactions 

Pregnancy / Lactation

Pregnancy consideration:  

No adequate data are available regarding the risk of drug in pregnant women. 

Breastfeeding warnings:  

quinidine is secreted in breast milk, but the secretion of dextromethorphan is unknown 

Pregnancy category: 

Category A: well-controlled and satisfactory studies show no risk to the fetus in the first or later trimester. 

Category B: there was no evidence of risk to the fetus in animal studies, and there were not enough studies on pregnant women. 

Category C: there was evidence of risk of adverse effects in animal reproduction studies, and no adequate evidence in human studies must take care of potential risks in pregnant women.    

Category D: adequate data with sufficient evidence of human fetal risk from various platforms, but despite the potential risk, and used only in emergency cases for potential benefits.    

Category X: Drugs listed in this category outweigh the risks over benefits. Hence these categories of drugs need to be avoided by pregnant women.    

Category N: No data is available for the drug under this category. 

Pharmacology

Pharmacology:  

dextromethorphan is a non-opioid derivative of the morphinan class, and its primary mechanism of action involves acting as a non-competitive antagonist at the NMDA (N-methyl-D-aspartate) glutamate receptors in the brain. It also acts as an agonist at the sigma-1 receptor. By blocking the NMDA receptors, dextromethorphan suppresses the cough reflex centrally in the brain, reducing the urge to cough. 

quinidine is an antiarrhythmic medication belonging to Class Ia in the Vaughan Williams classification. It blocks sodium channels in the cardiac tissue, prolonging the action potential duration and refractory period. This action helps stabilize the heart’s electrical activity and control abnormal heart rhythms (arrhythmias). 

Pharmacodynamics: 

The combination of dextromethorphan and quinidine addresses both neurological aspects of pseudobulbar affect (PBA) and enhances the pharmacokinetics of dextromethorphan. dextromethorphan’s NMDA receptor antagonism helps manage the neurological symptoms of PBA, while quinidine’s inhibition of CYP2D6 leads to increased plasma levels of dextromethorphan, thereby improving its efficacy in treating PBA episodes. 

Pharmacokinetics: 

Absorption 

dextromethorphan and quinidine are well absorbed from the GI tract after oral administration. 

Distribution 

dextromethorphan and its active metabolite, dextromethorphan, have a relatively large volume of distribution, indicating that they can distribute throughout the body’s tissues. Both substances can cross the blood-brain barrier, which is significant for their central nervous system effects. 

quinidine has a moderate volume of distribution, indicating that it is distributed in various body tissues. 

Metabolism 

dextromethorphan is extensively metabolized in the liver primarily by the CYP2D6 enzyme, which converts it into dextromethorphan, the active metabolite.  

Elimination and Excretion 

The elimination half-life of dextromethorphan is approximately 2 to 4 hours. After metabolism, its metabolites are mainly excreted in the urine. 

The elimination half-life of quinidine is approximately 6 to 8 hours. The primary route of elimination for quinidine and its metabolites is through the kidneys, which are excreted mainly in the urine. 

Adminstartion

Administration: 

dextromethorphan/quinidine combination is typically administered orally as capsules or tablets. The healthcare professional should provide specific dosing and administration instructions that the patient should strictly follow. 

Patient Information Leaflet

Patient information leaflet 

Generic Name: dextromethorphan/quinidine 

Pronounced as: DEX-troe-me-THOR-fan-and-KWIN-i-deen 

Why do we use dextromethorphan/quinidine? 

dextromethorphan/quinidine is a combination medication for treating pseudobulbar affect (PBA). PBA is a neurological condition characterized by sudden, uncontrollable crying or laughing that is often inappropriate or disproportionate to the individual’s emotional state. dextromethorphan is a common cough suppressant that acts on the brain’s cough center to reduce the urge to cough.