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You've probably seen it on TV or in a movie: a suspect is being interrogated about some crime and everybody knows that he did it but he won't confess, so an old man in glasses and a lab coat comes in with a syringe full of, like, bright orange stuff and he pokes it in there and the guy starts spilling the beans on everything, from the crime all the way to that weird dream he had about his mom in high school. (0:29)

They usually refer to this drug by a nice, science-y sounding name: sodium pentothal, and the crazy thing is: sodium pentothal is real. It's actually the brand name for sodium thiopental, and it really has been used as a so-called truth drug. Sodium thiopental was invented in 1936 by two chemists in Ohio who were trying to come up with a new painkiller. (0:48)
They were disappointed to find that the drug wasn't at all effective at killing pain unless it was given in really high doses at which point it just knocked out the patient entirely. Today, the drug is still used as a quick knock-out drug, like in the first stage of general anesthesia and even in lethal injections. (1:04)
But it's got another use. The chemists noticed that in smaller doses, sodium thiopental seemed to relax people, a lot. But not knock them out. That's because sodium thiopental is a barbiturate, a class of drugs that depresses the central nervous system and slows down brain activity. (1:18)
Now, at the time, psychiatrists had been using other types of barbiturates, like sodium amytal, to try to get patients to talk. Sodium amytal was commonly used during the first World War for example, to get soldiers to open up about traumatic memories from the battlefield. But even in relatively small doses, the drug was a powerful sedative, so it tended to produce, like, sleepier patients than talkative ones. (1:39)
By World War Two, psychiatrists were trying similar treatments with sodium thiopental, and soon, they found that in small doses it could induce a kind of trance-like state in traumatized patients, allowing them to talk about their experiences in a candid way that they never would when fully conscious. (1:52)
Eventually, some scientists began to think that if this chemical could help shocked soldiers talk, maybe it could have the same effect on criminal suspects. Years of research then followed that made some researches think that they truly had found a truth serum. And it's  not hard to see why they thought so. (2:06)
When sodium thiopental is injected, it ends up in your brain, where it imitates on of the chemicals that your brain naturally produces when it wants to slow things down, called GABA. Sodium thiopental binds with your brain cell's receptors for GABA and then just sits there. This keeps those cells from passing on signals which, when spread out over millions of neurons, has the effect of reducing higher brain function. (2:26)
This can help dial down the agitated states which come from fear and anxiety, but it also makes it really hard for the brain to carry out complex thought processes. Like, for example, deciding which questions to answer and making up lies. So for a while scientists figured that once you've lost the brain functions you need to tell lies, all you could do was tell the truth. (2:44)
But the real world isn't Hogwarts, and sodium thiopental isn't Veritaserum. Forcing someone to tell the truth is not as simple as flipping a switch. The problem with sodium thiopental is that by reducing people's ability to think complexly, it just makes them talk. A lot. About anything. (3:00)
In the 1950s, a forensic psychiatrist named John MacDonald reported that people under the influence of these drugs were extremely suggestible, and would say anything that they thought their interrogators wanted to hear, sometimes even confessing to crimes they hadn't committed. At the same time, MacDonald and others found that the drug still couldn't make unwilling subjects talk about the things they didn't want to, and people who truly believed their lies, like people with serious personality disorders, would just keep lying, no matter how much sodium thiopental they were given. (3:27)
Today, chemists and psychiatrists and cops will all tell you that these stories you've heard about truth serum... just are not quite true. (3:33)
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