During World War II, these drugs were used in a very different way. They were the first intravenous anesthetics and were used to treat traumatized soldiers who had lost their memories or had aphasia [loss of the ability to speak or process language due to brain injury]. Doctors found that using these drugs would make it easier for people to say what happened, and this helped them feel better.
As a result, a lot of doctors who had been in the military during the war were familiar with these drugs. Sodium amytal and pentothal were no longer just used as surgical anesthetics, although that was their most common use, but they were sometimes used for this psychiatric purpose of getting people to talk. In most cases, the drugs were not used in interrogations, but to help people talk about their memories in psychiatric consultations. However, some of these military doctors eventually became consultants for police forces or they did psychiatric research for the government and began exploring different ways of using these drugs for interrogation.
Do experts believe they really work?
The idea of a "truth serum" has never been widely accepted. Although there have been waves of enthusiasm for the idea of a drug that can extract information reliably, there has been even more skepticism. Ever since the 1920s, many judges, psychiatrists, and scientists have rejected the idea that there is a drug that can get memories out intact. They have claimed, instead, that it makes people feel like talking, but it also puts them in a state of extreme suggestibility: people will pick up on cues about what questioners want to hear and repeat that back. This is one of the reasons that statements made under the influence of these drugs have never, as far as I know, been accepted in an American court.
After 9/11, there were discussions in the national papers about whether it's a good idea to interrogate suspects using these drugs. Every time there is a desperate need for information from people, you get speculation about whether these drugs are going to get that information. But you also get consistent warnings that the information may be less reliable than what you would get without the drugs. That skepticism was there right from the start 80 years ago.