Use the search. 300mg of potassium cyanide are lethal already, 1g will do the job relatively quickly. If you do not get medical help administered immediately there's no rescue anymore. However cyanide is painful until you pass out. pass out should be quick after ingesting 1g of potassium cyanide.
This is my personal opinion.
Potassium cyanide needs much less quantity to b lethal and hardly any preparations (suggested meds) compared with the SN method. death occurs much quicker with cyanide than SN. It's probably more painful than SN until you pass out.
I consider cyanide as my personal emergency backup method, despite the possible painfulness for a few minutes.