A patient with chronic cough and sinus congestion notes constipation after taking a medication. Which ingredient is most likely responsible?

• A. Dextromethorphan
• B. Pseudoephedrine
• C. Guaifenesin
• D. Acetaminophen

Answer: (A)

Constipation from a medication given for cough points to an antitussive with opioid-like effects. Dextromethorphan is a common OTC cough suppressant that acts on the cough center in the brain, and although it has little analgesic power at typical doses, it carries μ-opioid–receptor–mediated effects. Those effects can slow gut motility, leading to constipation. The other ingredients—pseudoephedrine (a decongestant), guaifenesin (an expectorant), and acetaminophen (analgesic/antipyretic)—do not commonly cause constipation as a prominent side effect. Hence, the ingredient most likely responsible is the opioid-derivative antitussive.