50/50 chance of being painless/peaceful
60/70 chance of being successful
In case of failure possible: brain damage, multi organ damage (both also delayed possible after the person left the hospital),
destroyed immune system, cancer. The possible permanent damage depend on when you are found and when you get medical treatment. But as I said, damage can still occur later.
...
sanctioned-suicide.net
Better success/peacefulness ratio: one of the inert gas methods.