Toxicity Use of Nitrous Oxide for prolonged periods results in inhibition of the enzyme methionine synthase which is involved in DNA synthesis, causing changes in bone marrow after as short a time as 3-4 hours. This is a direct result of irreversibly oxidising the Cobalt II up to the III state in the Vitamin B12, a cofactor in the methionine synthase. Furthermore the enzyme cannot displace the oxidised B12, so the only regeneration possible is de-novo synthesis of new enzyme, in the presence of fresh, intact B12. Prolonged exposure to nitrous oxide may cause agranulocytosis, as well as leading to increased plasma concentrations of Homocysteine which has been implicated as a risk factor for peri-operative myocardial ischemia. Yo, found this on Wikipedia, thought you guys might want to check it out.