A nurse is caring for four clients. After administering morning medications, she realizes that a drug prescribed for one client was inadvertently administered to another client. Which of the following actions should the nurse take first?

The correct action for the nurse to take first is to assess the client's vital signs (Choice D). This is important because administering the wrong medication can have potentially harmful effects on the client's health. By assessing the client's vital signs, the nurse can quickly determine if there are any immediate adverse reactions or complications that need to be addressed promptly. This step ensures the client's safety and allows for timely intervention if necessary.

Reporting the incident to the supervisor (Choice A) is important but should come after assessing the client's condition. Gathering reversal agents (Choice B) may be needed depending on the situation but assessing the client's vital signs should take precedence. Notifying the client's provider (Choice C) is important but assessing the client's immediate status is more urgent.