Oxygen is widely available and commonly prescribed by medical and paramedical staff. When administered correctly it may be life saving, but oxygen is often given without careful evaluation of its potential benefits and side effects. Like any drug there are clear indications for treatment with oxygen and appropriate methods of delivery. Inappropriate dose and failure to monitor treatment can have serious consequences. Vigilant monitoring to detect and correct adverse effects swiftly is essential. Oxygen is often poorly prescibed. This drug chart was for a severely hypoxaemic patient with pneumonia and mild chronic obstructive pulmonary disease. The risk of carbon dioxide retention was secondary to the danger of severe hypoxaemia and inadequate oxygen was life threatening In a recent hospital survey 21% of oxygen prescriptions were inappropriate and 85% of patients were inadequately supervised. Similar studies report that oxygen is prescribed inappropriately in general practice. To ensure safe and effective treatment prescriptions should cover the flow rate, delivery system, duration, and monitoring of treatment. Tissues require oxygen for survival. Delivery depends on adequate ventilation, gas exchange, and circulatory distribution. Tissue hypoxia occurs within 4 minutes of failure of any of these systems because the oxygen reserves in tissue and lung are relatively small. The physiological and pathological mechanisms that result in tissue hypoxia will be discussed in later articles. They can be classified into two main groups: those causing arterial hypoxaemia and those causing failure of the oxygen-haemoglobin transport system without arterial hypoxaemia. More than one mechanism may contribute to tissue hypoxia, and predicting the response to supplemental oxygen requires careful evaluation of these functions. #### Checklist for safe prescribing of oxygen