While all members of the interdisciplinary team play important roles in the care of a client started on antidepressant medication, the psychiatric nurse typically takes a leading role in monitoring the medication's effectiveness and side effects. 

1. Pharmacist: The pharmacist plays a crucial role in ensuring the safe and accurate dispensing of medications. They provide important information about the medication, its dosing, and potential drug interactions. However, they may not be directly involved in monitoring the client's response to the medication or assessing for specific side effects.

2. Psychiatrist: The psychiatrist is responsible for prescribing the antidepressant medication and determining the appropriate treatment plan for the client. While they may periodically review the client's progress and adjust the medication as needed, their role may focus more on the overall management of the client's mental health condition rather than continuous monitoring of the medication's effectiveness and side effects. 

3. Psychologist: The psychologist typically focuses on providing therapy and counseling services to the client. While they may have knowledge about the effects of antidepressant medication, their primary role is not to monitor its effectiveness or side effects. 

4. Psychiatric nurse: The psychiatric nurse is often at the forefront of medication monitoring and management. They closely observe the client for changes in symptoms, assess the client's response to the antidepressant medication, and monitor for any potential side effects or adverse reactions. The psychiatric nurse may collaborate with the psychiatrist and other team members to adjust the medication regimen and provide ongoing support and education to the client about their medication.