Final answer:
The nurse should first assess the client's vital signs after administering the wrong medication, followed by notifying relevant healthcare professionals and documenting the incident.
Step-by-step explanation:
If a nurse administers a dose of metformin to a client instead of the prescribed dose of metoclopramide, the first action the nurse should take is to assess the client's vital signs. This immediate assessment is critical to determine if the client is experiencing any adverse effects from the incorrect medication. After evaluating the client, the nurse should then take further necessary actions which include notifying the healthcare provider, informing the charge nurse, and documenting the error in the medical record.