Which of the following describes a side effect?

The statement that describes a side effect accurately captures the essence of what a side effect is. A side effect refers to an unexpected reaction that occurs alongside the desired therapeutic effect of a medication. It is typically not the primary reason for prescribing the medication and may arise independently of the drug's main therapeutic action. For instance, while a pain reliever is intended to alleviate pain, it might also cause drowsiness; this drowsiness is an example of a side effect.

In contrast, the other choices do not fit the definition of a side effect. A desired effect of a medication refers to the main or anticipated outcome for which the medication is prescribed, not a side effect. An allergic reaction can occur as a result of exposure to a medication, but it is classified separately as an allergy and not a general side effect. Lastly, a result of incorrect dosage involves administering too much or too little of a medication, which can lead to adverse effects or insufficient therapeutic effects, but again, this situation doesn't embody the concept of a side effect as it relates to the medication's intended outcomes.