Animals are "put to sleep" with a single large dose of barbiturates, but this is not how humans are executed. Execution by lethal injection is done with a complex mixture of compounds, generally sodium thiopental (a very short acting barbiturate), potassium chloride, and paralyzing agents which are no longer legal to use on animals because of the length of time they take and their potential to cause suffering. Because of the drugs and the process used, it is supposed to take between 2-10 minutes for the person to die, but it can take considerably longer than this, and the longest time on record is 24 minutes between the initial injection and the time of death, during at least part of which (after the first few minutes) the prisoner was probably fully conscious. University of Miami researchers, among many others, have made the recommendation that the prison system look to the American Veterinary Medical Association's recommendations for humane euthanasia, but possibly because of the difficulty and time that it would take to change the protocols, the US federal courts have used a number of procedural devices to avoid having to rule on the complaints. There is more information at an interesting Wiki: http://en.wikipedia.org/wiki/Lethal_injection