Final answer:
The client experiencing restlessness and an urge to move while on antipsychotic medication is likely suffering from akathisia, a common side effect of 1st generation antipsychotics.
Step-by-step explanation:
The client who has schizophrenia and has started taking a 1st gen antipsychotic medication 3 weeks ago is likely experiencing the adverse effect known as akathisia. Akathisia is characterized by feelings of inner restlessness, an urge to move, rocking back and forth while sitting, and pacing. This is a common side effect associated with 1st generation antipsychotics due to their dopamine antagonist actions, which can lead to extrapyramidal symptoms like akathisia. These symptoms are significant as they can be extremely distressing to the patient and may impact their compliance with the medication regimen.