The most frequently used substance in a medical emergency is:

The most frequently used substance in a medical emergency is oxygen, which plays a critical role in treating various conditions, particularly those that involve respiratory distress or hypoxia. Oxygen is essential for cellular respiration, and when the body's oxygen levels drop, it can lead to serious complications. In emergency situations such as acute asthma attacks, cardiac events, or any condition where the airway is compromised, administering oxygen helps to quickly restore adequate levels of oxygenation to the patient’s bloodstream.

In many emergency protocols, oxygen is one of the first interventions employed due to its effectiveness in improving respiratory function and overall patient stability. It is non-invasive and can be administered rapidly, making it a vital resource for healthcare providers in emergency settings. This highlights the importance of oxygen in ensuring the patient's immediate needs are met before further interventions may be required.