Pure bullshit!
Nitrous is NOT like inhaling glue or freon or gasoline fumes or something.
Nitrous Oxide is one of the absolute safest and NON-TOXIC substances that there is. 99.9% of it is exhaled unchanged. And again, it is extremely non-toxic.
Why do you think it is used in dentistry when you would be laying there inhaling it for like an hour straight?
See here:
http://www.erowid.org/chemicals/nitrous/nitrous_health.shtml
and here:
http://en.wikipedia.org/wiki/Nitrous_oxide
The thing about B-12 depletion has only been observed a couple times when people did truly immense amounts (like an entire tank over 8 hours), and is entirely prevented by typical doses of B-12 in multivitamins.
Only ignorant jackasses tell people it is toxic without knowing what the hell they are talking about.
One the other hand if you are stupid enough to use Industrial or Automotive Nitrous Oxide, that has sulfur deliberately added to it to discourage abuse, then YES, THAT shit can be damaging.
Moral: use only medical-grade nitrous from tanks, or food-grade nitrous from whippets and you have absolutely nothing to worry about.