Final answer:
The owner should be informed about the possible side effects of diphenhydramine, which can include sedation, dry mouth, urinary retention, and gastrointestinal disturbances in their dog.
Step-by-step explanation:
The veterinarian has prescribed diphenhydramine preoperatively to a golden retriever undergoing a mast cell tumor resection. Diphenhydramine is an antihistamine that works by preventing mast cells from releasing histamines, as well as blocking the action of histamines at the receptor sites, thus reducing inflammation, itching, and allergic reactions.
While it is an important medication for this procedure, some potential adverse effects that the owner should be aware of include sedation, dry mouth, urinary retention, and gastrointestinal disturbances. It's critical to discuss these possible side effects with the owner to ensure they can monitor their pet post-operatively for any significant symptoms that may develop.