Cyanide is known as a highly toxic chemical, and can certainly cause death at the right concentrations. However, purchasing it is nigh on impossible, although on the Peaceful Pill Handbook forum (for PPH members only) there was one report of it being sold from China. It is possible to manufacture it, although not easily. In The Peaceful Pill Handbook1 there is detailed information on how to manufacture sodium cyanide. There are also a few articles that can be found searching on the web, although less detailed.
Manufacturing sodium cyanide requires extreme care, and it is certainly not recommended using the manufactured product without testing it first for lethality, otherwise the results could be extremely painful.
It is also possible to combine sodium cyanide with concentrated hydrochloric acid to create hydrogen cyanide gas that can be lethal in an enclosed space, and this method has been used in gas chambers over the years.
It should be noted though, that in the US, carrying out the death penalty by use of lethal gas resulted in many reports of suffering during executions. Some executions saw the prisoner take up to 11 minutes to die, and be in agonising pain during that time. This method was therefore abandoned as the primary method of execution in the US, and in California was labelled as being "unconstitutionally cruel".
Other reports of death by cyanide (whether ingested or gas) are also very mixed as to whether they are pain free. In Final Exit2, Derek Humphry quotes examples where death was swift, and others where it was "miserable and violent". Humphry concludes that on balance this is a method "best not used".
Potassium chloride (injected) is also said to be potentially lethal, and more accessible as a method (further information on this method (and cyanide) is available in the book by Stone3). Working out the minimum lethal dose can be tricky though. One report on the alt.suicide forum said "I injected potassium chloride (50mg/kg) and it wasn't lethal but it burned like hell, now I have blisters, my arm is swollen and the pain is beyond belief". Combining the method with sedatives and muscle relaxants might help.
However, this method is complicated to execute and potentially unreliable with potential to cause significant pain and possible long term health issues.
As a result, careful consideration should be given before contemplating using either of these poisons as a method of suicide, especially if a peaceful and painless death is desired.
Dr Phillip Nitschke with Dr Fiona Stewart, The Peaceful Pill eHandbook (www.peacefulpillhandbook.com), revised November 2011.
Derek Humphry, Final Exit, 2010.
Geo Stone, Suicide and Attempted Suicide, 1999.