Nitrous oxide (N2O) has been used as an anaesthetic agent since 1844. It has two well recognized adverse effects: it oxidizes Co(I) to Co(II) and so inactivates vitamin B12, and its high partial pressure in blood allows it to expand into cavities in the body such as the pleural space. Impairment of the action of vitamin B12 can cause both megaloblastic anaemia and, especially in recreational users of N2O, the syndrome of subacute combined degeneration of the spinal cord. The overall cardiovascular effects in noncardiac surgery seem to be minor. N2O is probably a human teratogen.