It does, however, say: > the clinical manifestations of vitamin B12 deficiency have been noted not only in long-term nitrous oxide users, but also after a sing

It does, however, say: > the clinical manifestations of vitamin B12 deficiency have been noted not only in long-term nitrous oxide users, but also after a single exposure in those with susceptibility to vitamin B12 deficiency Basically, there are several sources of susceptibility, N2O being one of them, and they add up.