medicine and dentistry. It can therefore be used safely in most patients in the dental setting. However, in some patients sedation with nitrous oxide would be unsuitable. Some examples are below. A patient who is unable to use a nasal mask. This is considered an absolute contraindication. Inability to use a mask can either be due to anatomic and/or disease-induced nasopharyngeal obstructions, or due to psychological and/or cognitive disturbances that lead to intolerance of mask placement. Examples of such patients include those with upper respiratory infections; severe sinusitis; and uncooperative paediatric patients or those with a fear of nasal masks. Patients who have recently undergone ear surgery. This is because any condition that compromises the patency of the eustachian tube increases the likelihood of pressure build up in the middle ear in the presence of nitrous oxide. Patients diagnosed with certain psychological disorders for example; schizophrenia or bipolar disorder. This is a relative contraindication because nitrous oxide sedation has been successfully used in patients with other personality and psychiatric disorders. The key concern with these patients is the use of nitrous oxide in addition to the many other psychotropic agents that they are already taking. This may further alter such patients’ disposition and lead to highly unpredictable results. Prior medical consultation is highly recommended for this group of patients. Pregnant patients. This is also a relative contraindication because retrospective studies have failed to identify any adverse outcomes for either the patient or the foetus. However, nitrous oxide sedation, like any other elective dental treatment, should be avoided during pregnancy. This is especially true in the early weeks of pregnancy to minimize the risk of possible toxicity to the developing embryo. Necessary/emergency dental care that requires the use of nitrous oxide to reduce stress should not be denied to a pregnant patient. Pre-cooperative patients. This is because the patient must understand they must minimize talking and mouth breathing to facilitate proper sedation. Patients with respiratory conditions such as COPD. These patients depend partly on a low blood oxygen to initiate a breathing stimulus, so inhalation sedation removes that hypoxic drive. Problems Inhalation sedation with nitrous oxide/oxygen is a well recognised safe and effective technique. The desirable effect of the procedure is to relax the patient while maintaining verbal contact. Compared to other sedation agents, it causes minimal physiological stress to the patient and risk of loss of consciousness is less likely.