Which of the following side effects is associated with decongestants?

Decongestants are known to primarily constrict blood vessels in the nasal passages to reduce swelling and congestion, which can lead to certain side effects. One well-documented side effect of decongestants is anxiety. This occurs because these medications often stimulate the central nervous system. As a result, individuals may experience increased alertness, restlessness, or feelings of anxiety, especially when higher doses are taken or in sensitive individuals.

Other options, while they may seem plausible in some contexts, are not typically associated with decongestants. For instance, sleepiness is more commonly linked to antihistamines rather than decongestants, while nausea and rash could arise from various medications, but they are not classic side effects of decongestants specifically. Thus, anxiety stands out as a well-characterized effect that can result from the use of decongestants.