Final answer:
Weight gain is a recognized adverse effect of amitriptyline, a tricyclic antidepressant, which can occur when treating major depressive disorder.
Step-by-step explanation:
A nurse assessing a client with major depressive disorder who is taking amitriptyline should recognize that an adverse effect of the medication can include weight gain. Amitriptyline is a tricyclic antidepressant, and these medications can contribute to various side effects. Common side effects of antidepressants, and particularly tricyclic antidepressants like amitriptyline, include weight gain, fatigue, dizziness, and dry mouth. Although it may seem counterintuitive, given that depression can lead to weight loss, the pharmacological treatment with tricyclic antidepressants often results in the opposite effect.