The long held belief that oxygen (O2) is always beneficial for ill and injured patients is being challenged by recent studies suggesting that excessive oxygenation may result in free-radical mediated tissue damage, resulting in increased morbidity and mortality. Recent guidelines suggest that O2 should be 'titrated' to achieve an appropriate target saturation of 94-98% rather than being administered in an all-or-nothing fashion, particularly in patients with myocardial ischemia or those following resuscitation from cardiac arrest. The Scottish Intercollegiate Guidelines Network (SIGN), British Thoracic Society, National Institute of Health Care Excellence (NICE), European Resuscitation Council and other respected bodies all restrict O2 use to those with hypoxemia, pulmonary oedema or continuing myocardial ischemia, based on expert opinion and pathophysiological reasoning rather than hard evidence.