The present invention is a method and system for administering nitrous oxide. The system includes a fluid control system for controlling the flow of nitrous oxide and oxygen to the patient. A nitrous oxide and oxygen supply, vacuum source, a breather bag, and a nasal delivery interface system are fluidly connected to the fluid control system and a patient. The fluid control system also includes a safety scavenge system including a mass airflow sensor, master controller, nitrous oxide valve, and alarm. The mass airflow sensor reads the scavenging vacuum pressure which it communicates to the master controller. Depending upon the scavenging vacuum pressure, the master controller can activate an alarm or shut off the flow of nitrous oxide. In operation, the present invention provides a system for administering anesthesia/analgesia gas which prevents excessively high volumes of exhaled nitrous oxide in the operatory environment through monitoring of the scavenge vacuum pressure.