SN stands for Sodium Nitrite, which is more properly called NaNO2 (with a little 2). SN is a slightly more understandable name. It is not to be confused with Sodium Nitrate, which is a different chemical.
There is a guide to it here:
This post will be dedicated to compiling all the information we have on the Sodium Nitrite method (SN), including what else is needed and comparing it to other popular methods. Before starting, I HIGHLY encourage you read most, if not all, of the chapters on the PPH, as this method may not be...
sanctioned-suicide.net
The advantages of SN, which I am here stating without suggesting anything about suicide:
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Accessibility. SN is not
easy to get hold of, but it isn't especially
difficult either. It is not a controlled substance the way that, say, heroin is. Some of the recommended "supporting" drugs can be harder to get hold of, especially benzodiazepines which are controlled substances.
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Privacy and non-violence. An SN suicide can take place in private, either in the comfort of your home or in a hotel room. This is attractive to people who do not want to commit suicide in a public place. It is also non-violent, a term used broadly to refer to any death resulting from force on the body. While there is the tragedy for surviving friends and family, and some trauma for whoever finds the body, it doesn't "involve" someone in your death the way that some methods do, and there are not "bystanders".
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Low risk of non-lethal harm. Unlike some more peaceful methods, if a SN suicide is interrupted then it is unlikely you will suffer permanent damage unless you have progressed quite far.
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Low technical competence. Compared to some other methods, SN is relatively foolproof. There are some potential barriers for people who cannot weigh things, but most people can.
This is balanced by:
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Moderate discomfort. SN is not painless, with most people who take it experiencing nausea and vomiting, even with the use of antiemetics, as well as other symptoms like tachycardia, headaches, and sometimes shortness of breath.
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Low reliability. SN can kill at fairly low levels but a sizeable proportion of attempts fail. This seems to be primarily because the attempter regrets their decision and calls out for medical treatment. This interacts with "low risk of non-lethal harm" - it's somewhat more acceptable for SN to be slightly less reliable than other methods because the consequences of a "failure" are now. That said, SN should not be used for "suicidal gestures" as it is still very deadly and clinicians may not have the antidote to hand.