That is probably one of the least painful and quickest way to put to sleep. It is at-least quicker than hanging, electrocuting and venomizing.
On a side note, do you know what happend in USA when they used their latest lethal injection to put a person to permanent sleep? The person stayed active for ~35 minutes, even tried to sit down and talk. Then they closed the witnesses' view to the scene and did "something" to him. Victim died after roughly 55 minutes and receiving 2 major heart-attacks. That was a "quick and least painful way" of putting somebody to sleep - NOT!
See the latest execution lasted
18 minutes. Crazy as it can possibly be.
Charles Warner Executed: Baby Killer Says 'My Body Is On Fire' During Lethal Injection In Oklahoma
Want to know about electrocuting? You probably don't!