Oxygen is probably the most common drug to be used in the care of patients who are present with medical emergencies. Supplementary oxygen therapy is required for all acutely hypoxemic patients and for many other patients who are at risk of hypoxemia, including patients with major trauma and shock. Most acutely breathless patients will require supplementary oxygen therapy, but there are some situations, such as acute hyperventilation or diabetic ketoacidosis, where an apparently breathless patient will not benefit from oxygen therapy.