I remember this thread triggered a little discussion about nitrous oxide, and apart from use for medical reasons, it appears one or two peeps used it recreationally as well.
I've got to refresh my medical gases certificate soon and I thought it would be wise to do a bit of revision on the contraindications relating to nitrous oxide use for pain relief (as "entonox") as I couldn't recall all of them off the tip of my tongue. I listed a few of them on that earlier thread.
In doing so, I came across a set of clinical guidelines
http://www.ich.ucl.ac.uk/clinical_infor ... ionals6519
Entonox should also be used with caution in patients with poor nutritional status i.e. those:...
...with a poor intake or on a diet low in animal products e.g. vegans
Having not heard this one before, and intrigued (if not a bit irritated by the wording) I decided to back check their rationale.
In doing so I came across some literature from the 1990s (e.g. http://jnnp.bmj.com/content/60/3/354.1.full.pdf) which suggests that entonox is in fact, dodgy for people with low B12 status. It seems that nitrous oxide can oxidize the cobalt in cobalamin to an inactive form which reduces the activity of cobalamin-dependent enzymes. In people who aren't getting B12 regularly enough, this can cause some serious problems quite quickly. In the case report I linked to, the patient (a "strict vegetarian") suffered spinal cord degradation with some bad problems up to a year after being operated upon for a hip fracture using entonox for anaesthesia.
This is not so much an argument for not fooling around with this gas, as there are plenty of other good reasons, or a specifically vegan thing, for obvious reasons.
But, it might be worth adding it to the manifold reasons to supplement with B12, and to pay extra attention to it if you know you're going to be exposed to entonox (e.g. for surgery or childbirth), or ASAP afterwards (being hit by a bus). And perhaps have a chat with your doctor if you're concerned.