Final answer:
The hospital should first change the access code to reduce immediate safety risks in the situation.
Step-by-step explanation:
In order to reduce immediate safety risks in this situation, the hospital should first change the access code. By changing the access code, the hospital can prevent unauthorized access to restricted areas and medications, thereby ensuring the safety of patients and preventing further harm. Disciplining the nurse and performing a pharmacy inventory can be important steps to address the incident, but changing the access code should be prioritized to immediately mitigate safety risks.