What is a potential adverse reaction to dextromethorphan?

Dextromethorphan, a common over-the-counter cough suppressant, can indeed lead to nausea as a potential adverse reaction. Nausea can occur as a result of the drug's action on the central nervous system. While it is generally well-tolerated, some individuals may experience this gastrointestinal side effect, especially if they exceed the recommended dosage or have a sensitivity to the medication.

It’s important to understand that while the other options may also occur with various medications, they are less commonly associated with dextromethorphan specifically. Constipation is more frequently linked to opioid medications, rash can arise from various allergies or sensitivities but is not a primary concern for dextromethorphan, and insomnia may occur but is not a typical reaction associated directly with this cough suppressant. Thus, nausea is the more prominent adverse effect to be aware of when using dextromethorphan.