Dextromethorphan (DM) is a common, over-the-counter medication found in numerous cough and cold preparations. It is primarily used to relieve a persistent, non-productive cough, and is generally considered safe when taken as directed. Despite its widespread use, the relationship between this popular antitussive agent and changes in blood pressure is a frequent concern for users with pre-existing health conditions.
Understanding Dextromethorphan
Dextromethorphan is classified chemically as a morphinan derivative, sharing a structural similarity with certain opioids, but functioning differently in the body. It serves as an antitussive, suppressing coughs by targeting the central nervous system. DM concentrates its effects on the cough center, located within the medulla oblongata of the brainstem.
By modulating activity in this area, the drug raises the threshold required to trigger a cough reflex, reducing the frequency of coughing episodes. DM is metabolized in the liver by the CYP2D6 enzyme. While structurally related to opioids, DM does not produce typical analgesic or addictive properties at standard therapeutic doses.
Dextromethorphan’s Direct Impact on Blood Pressure
When taken at the recommended therapeutic dose (typically 15 to 30 milligrams every four to six hours), DM does not usually cause a significant elevation in blood pressure for most healthy individuals. Any minor, transient increase in blood pressure at standard doses is clinically insignificant.
However, the risk changes substantially in scenarios involving high doses or misuse, where the drug’s other pharmacological activities become more pronounced. In cases of overdose or abuse, DM toxicity can manifest with cardiovascular symptoms, including hypertension and tachycardia. This severe reaction is related to DM’s broader effects on the central nervous system at very high concentrations.
Many over-the-counter cold medicines containing DM are combination products. These often include decongestants like pseudoephedrine or phenylephrine, which are known to directly raise blood pressure by constricting blood vessels. If a blood pressure increase occurs while taking a combination product, the decongestant ingredient is typically the primary cause, not the dextromethorphan itself.
The Role of Drug Interactions
The most significant risk of a severe blood pressure spike occurs when DM is combined with other specific medications. This interaction is linked to the drug’s weak activity as a serotonin reuptake inhibitor. Combining DM with other medications that increase serotonin levels can lead to serotonin syndrome, a potentially harmful condition characterized by autonomic dysfunction and blood pressure fluctuations.
Monoamine Oxidase Inhibitors (MAOIs)
The highest risk of dangerous blood pressure elevation, potentially leading to a hypertensive crisis, is seen with Monoamine Oxidase Inhibitors (MAOIs). MAOIs are a class of prescription drugs used for depression and Parkinson’s disease. Since MAOIs prevent the breakdown of neurotransmitters, combining them with DM causes a rapid and excessive buildup of serotonin, leading to severe hypertension and hyperthermia. Individuals must observe a mandatory two-week washout period after discontinuing an MAOI before using DM.
Other Serotonergic Agents and CYP2D6 Inhibitors
A similar, though less severe, risk exists when DM is co-administered with Selective Serotonin Reuptake Inhibitors (SSRIs) or Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs). These common antidepressants also increase serotonin levels, creating a synergistic effect with DM that may result in serotonin syndrome. Furthermore, medications that inhibit the liver enzyme CYP2D6 can dangerously amplify DM’s effects. This enzyme metabolizes dextromethorphan, and its inhibition prevents DM clearance, leading to toxic accumulation and increased cardiovascular instability.
Signs of Concern and When to Seek Medical Advice
Recognizing the signs of an adverse reaction is important, as symptoms of dangerously elevated blood pressure often overlap with those of serotonin syndrome. Autonomic symptoms suggesting a severe reaction include a rapid heart rate, excessive sweating, and notable changes in blood pressure. Neuromuscular signs may include muscle rigidity, tremor, and overactive reflexes.
A sudden, severe headache, blurred vision, or confusion are specific red flags that warrant immediate attention. If you experience these severe symptoms, especially while taking other medications, stop using the dextromethorphan product and seek emergency medical care. For routine concerns, such as mild dizziness or nervousness, consulting a pharmacist or primary care physician is appropriate. Always review all prescription and over-the-counter medications with a healthcare professional before starting dextromethorphan to identify potential drug interactions.