What classification includes undesirable effects that occur alongside the desired therapeutic effect?

The classification that includes undesirable effects occurring alongside the desired therapeutic effect is side effects. Side effects are defined as secondary, typically undesirable effects that may occur in addition to the primary therapeutic effect of a drug. For instance, a medication intended to relieve pain may also result in drowsiness, which is a side effect. These effects are often known and anticipated based on the drug's pharmacological profile.

In contrast, allergic effects result from an immune response to a drug, while toxic reactions indicate harmful effects that can potentially cause serious harm or lead to overdose situations. Adverse reactions may encompass both side effects and allergic reactions but are generally considered to be more severe and can be unpredictable. Recognizing side effects is crucial in clinical practice, as they can impact patient adherence and overall safety.