A barbiturate (a drug that acts as central nervous system depressant, i.e. sodium thiopental or propofol) in combination with a muscle relaxant. Death cause: oxygen deficiency, within seconds.

Source: https://nl.wikipedia.org/wiki/Euthanasie (dutch)

Minor correction propofol is not a barbiturate. They also tend to use longer acting barbiturates. Sodium thiopentol wears off after 10-15 minutes.