Cecil reports that " by the 1920s scopolamine had become the first drug billed as a truth serum." He goes on to say that interrogators found that the side effects of a very dry mouth, unconducive to free talking, made the drug more trouble than it was worth.
He leaves unquestioned though the claim of the Texas doctor who pioneered its use in interrogating criminal suspects that “it impaired reasoning enough to make lying impossible.”
part of the hysteria started when all the tv crime shows put out the idea that if you blew a handful in someones face and eyes youd get the "zombie"effect …
More generally, even if we limit “truth” to “things the speaker believes to be true” (see current events for why that’s not all that useful), there’s no such thing as a “truth serum” that can reliably produce honesty, Hollywood notwithstanding. If there was, we wouldn’t even be having the discussions about the use of torture on our enemies, and the court system would be very different than it is today. Lying is wired pretty deeply into us, and it’s not possible to completely shut it down without shutting down cognition itself.
The best research around technical “lie detection” these days requires active scanning of the brain during interrogation, along with a good set of baseline data for what dishonesty looks like in that particular individual. It’s not impossible that this technique will be developed to a reliability level that gives us reasonably true lie detection in the next few years or decades, but the “jab a needle in their neck and they’ll tell us their whole plan” type will remain fiction.
Right now, the best lie detectors are people. That’s basically the polygraph argument in a nutshell: the machine itself is worthless, the person running it is usually trained to do better than chance at reading people via mannerisms, expressions, etc. But “better than chance” isn’t the same–isn’t anywhere near the same–as “reliably.”