Which of the following findings indicates an adverse effect of amitriptyline?

Amitriptyline is a tricyclic antidepressant that can have various side effects related to its anticholinergic properties. Urinary retention is a common adverse effect that can occur due to its action on the cholinergic system, which inhibits bladder muscle contraction and can lead to difficulty in urination. This side effect is significant because it not only affects the patient's comfort and daily activities but can also lead to more serious complications if not addressed.

In contrast, muscle twitching may be associated with various factors but is not a well-documented primary adverse effect of amitriptyline. Cough is not directly related to the pharmacological effects of amitriptyline, and while increased libido is generally less common with antidepressants, it is not specifically recognized as an adverse effect of this medication. Thus, urinary retention stands out as a clear and recognized adverse effect of amitriptyline.