As far as I know, animals at the pound are not euthanized by injection. That would be WAY too expensive. I believe the methods used are gas, freezing, and vacuum chamber. Of those, I believe the most common is vacuum chamber. They simply put a bunch of animals in a sealed chamber and suck the air out. It's quick and cheap.

Sadly, the answer is no. San Antonio Express-News – 2004 - Texas The cost of closing the gas chambers at the San Antonio animal pound and using lethal injections for euthanasia could be a fraction of the city's estimate, according to a standard analysis used by other municipal pounds. Metro Health Director Dr. Fernando Guerra told city leaders at a packed public forum last week that it would cost as much as $500,000 for equipment and additional staff to administer the injections. That would increase the pound budget of $3.2 million by 16 percent, a significant expansion for an agency that has been a low city priority and has had its staffing trimmed in the past five years. But the Express-News, using a formula relied on by animal-control departments around the country, found the annual cost of lethal injections at the pound would be no more than $74,000 — including startup and staffing costs — to euthanize nearly 50,000 cats and dogs, the same number that will be gassed this year. The Express-News could not find out by the end of the week how much the pound spends on its three gas chambers for comparison. "The bottom line is the number is almost the same. It's essentially a wash," said Douglas Fakkema, a nationally recognized expert in animal euthanasia who long has advocated injection as the most humane method of putting down animals. http://www.mysanantonio.com/news/metro/stories/MYSA120504.1B.pound_costs.c0786e5b.htm Very sad that some areas of the United States still use gas chambers for euthanasia. The above site speaks only of the cost effectiveness and not about the inhumane way they discard life.

I think this depends on where you are in the world because I am in Canada and they always use injection methods because it is the safest and most humane. I am however sure that there are many other methods used other then just injection, as the other two have written .