Nitrous oxide brain damage
I've heard a lot of bogus reasons for why people don't do nitrous oxide (the top of the list being "Yeah, maaaan - that buzz you feel is your brain cells dying!" and "Nitrous makes holes in your blood-brain barrier"), and I've also seen some astonishingly bad nitrous inhaling practices. While I could be just another stroker rehashing old myths, I've tried to delve a little deeper into the science of nitrous. If you find anything erroneous in this passage, please let me know (email@example.com - I think all you humans can figure out what to do with that address to make it correct).
First, let's address the issue of how nitrous works. That buzz you feel is NOT due to deprivation of oxygen. We all know that if you hold your breath or inhale carbon monoxide (don't do that!) you do not get a nice buzz. You are likely to merely get a headache, pass out, or die, depending on the severity of your actions. What is going on is that the nitrous oxide, because it is fat soluble (which is incidentally why they use it for making whipped cream), crosses the blood-brain barrier and tingles some receptors in your head. [If any of you with more education can provide specifics, that would be great.]
As is the case with any substance that's fun, nitrous is not completely safe, either. As far as I can tell, there are three main ways nitrous can harm your brain: vitamin B12 deficiency, hypoxia, and brain lesions.
One side effect of inhaling nitrous is that each time, a little bit of your body's supply of vitamin B12 gets oxidized. Vitamin B12 is essential for the myelination of the nerves in your body. Without vitamin B12, your nerves de-myelinate and you get neural degradation. This has happened in a couple of cases to hard-core vegans who didn't do all their research and failed to eat enough B12. If you're eating well and keeping healthy, a little less vitamin B12 is not a problem at all. However, if you're oxidizing it faster than your body can absorb and/or utilize it, then you have a problem. How much nitrous does this take? A _lot_. There is one (in)famous case where a man did 200 whipets a day for 6 months. He developed a loss of motor coordination (read: had problems walking) and urinary incontinence. What happened to his brain was probably not pretty, either. Don't try that at home, kids.
This oxidization phenomenon is also why dentists who use nitrous regularly may, over the course of months, slowly develop numbness or tingling in their faces, even though they are not inhaling enough nitrous at once to get anesthetized or high.
Another mechanism by which nitrous can harm you is hypoxia (low oxygen levels in your blood). There are two somewhat orthogonal ways you can get yourself into this situation. One is to not breathe any oxygen - you could inhale balloon after balloon of helium, for example. Contrary to what most people think, your body's reflex to breath is not triggered by lack of oxygen, but rather by the build-up of carbon dioxide in your blood. That's why it's pretty easy to kill yourself with misconfigured scuba gear.
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