I suspect this is morphine, or at least supposed to be. For a very long time morphine was the painkiller of choice, and ESPECIALLY for soldiers in the first and second world wars, to the point where they were issued it in first aid kits. It was fast acting, (relatively) easy to administer, and extremely effective. Unfortunately it was also incredibly addictive (heroin literally turns into morphine in the body), which is why it's so heavily controlled in modern days. Its use was so prevalent in the military that morphine addiction became known as "the soldier's disease" following WW1.