'Truth serum' drugs do exist. Here's how medicines like sodium pentothal and scopolamine can manipulate the brain.

• The term "truth serum" refers to a number of mind-altering drugs.
• These drugs make you incapable of lying, or so the theory goes.

Humans have known since the time of the Roman Empire that we're more readily truthful while under the influence. That's where the idea behind the term "truth serum" comes from.

Truth serum refers to a number of mind-altering drugs that are supposed to make you incapable of lying, but the reality is no drug is powerful enough to grip the human mind so tightly as to make it impossible to lie.

Some truth serums, like sodium thiopental, slow the speed at which your body sends messages from your spinal cord to your brain.

As a result, it's more difficult to perform high-functioning tasks, such as concentrating on a single activity like walking a straight line or even lying. It's this concentration you need to think up a lie that truth serum removes. So in that way, lying can be more difficult, but not impossible.

The same thing happens when you're nodding off and reaching that twilight state where you're in between consciousness and sleep. If you're not a compulsive liar, then it's likely more difficult for you to lie than tell the truth.

As the famous American author wrote in "Mark Twain's Notebook" (published posthumously in 1935): "If you tell the truth, you don't have to remember anything."

That said, there's no way to really know if someone is telling the truth, ever.

Numerous accounts and scientific reports suggest that you're more prone to tell the truth under the effects of truth serum drugs, but the drugs have other side effects. Most concerning among them is that they might make you say something to please someone else, even if it's not true.

Furthermore, not only are truth serum drugs not all that useful, they are illegal under certain circumstances, including interrogation.

Although many of the first drugs that the CIA, police, and Nazi interrogators used throughout the '20s, '30s, and '40s are still around today, they have other uses, including ingredients in medicines that prevent motion sickness and for lethal injection.

Here's a list of the most common mind-altering, manipulating medicines that have used as truth serums.

Sodium Pentothal

Sodium pentothal is a type of barbiturate, which is a series of drugs that are central nervous system depressants, colloquially known as "downers." Downers slow your body's process to transmit information to your brain and are common prescription medicines for pain relief, sedation, muscle relaxation, and lowering blood pressure.

An overdose of barbiturates can be lethal and has led to a number of celebrity deaths, including Marilyn Monroe, Judy Garland, and Jimi Hendrix. The drug was also one of the first used in lethal injections in the US and is most often administered intravenously.

Until 2011, it was sometimes used as an anesthetic because patients usually pass out within 30-45 seconds after taking the drug. But the US has stopped using the drug.

In 2011, the Italian company that produced the drug announced it was ceasing production, The Guardian reported at the time. The company was worried that Italian authorities would use it in executions and as a result, the US lost its only viable supplier.

There are still accounts of this drug's use as a truth serum. In 2007, police in New Delhi, India administered sodium pentothal to a wealthy businessman, Moninder Singh Pandher, and his servant, Surinder Koli, who were suspects in the infamous Noida serial murders. While under the influence, they confessed to luring children to their home, raping, and then killing them. The servant, Koli, was given the death sentence and is still in jail, and Pandher received a seven-year sentence.

Scopolamine

Scopolamine was first promoted by Dr. Robert House as a truth serum in the early 20th century, and was the first drug to adopt the name "truth serum."

Throughout the 1920s and '30s some police departments in the US used scopolamine on suspects, and — in some cases — judges permitted the statements the subjects gave up while under the influence.

Scopolamine was the truth serum drug of choice for many back in the day because it also wiped a subject's memory clean so they didn't know what they said after waking up, Gizmodo reported.

The drug comes from the seeds of a tree, which locals in Colombia where it grows call the "get-you-drunk" tree. While some Nazis used it in interrogation, today it's in many medicines to help prevent motion sickness and tremors of Parkinson's disease. It's also used as a date-rape drug.

Scopolamine can be ingested orally through a pill, or in one bizarre case, as a rub. Reports recount three young women in the Colombian capital, Bogota, who smeared the drug on their breasts luring men to lick it off. Once the men were incapacitated, the women would drain their bank accounts.

Sodium Amytal or Amobarbital

Sodium Amytal is also a type of barbiturate, or downer. It was widely used during World War II as an anti-anxiety drug for soldiers with the psychological disturbance called shell shock.

But like all truth serum drugs, sodium amytal is a powerful sedative, and that side effect combined with the dis-coordination and cognitive impairment it induces is why soldiers stopped using it.

Moreover, sodium amytal is highly addictive. This drug is sometimes used to treat insomnia and is often administered intravenously, although it can come in powder form for oral ingestion.

Take too much of this drug and it can be lethal. The maximum dose for an adult is one gram.

This drug is no longer used as a truth serum because subjects sometimes develop false memories after the fact.

Ethyl Alcohol

That's right. Booze!

The Italian phrase "In vino veritas," which is Latin for "In wine there is truth", is attributed to a Roman philosopher known as Pliny the Elder.

So, humans have known for roughly 2,000 years about alcohol's ability to loosen the tongue.

Whether you're drinking it down or taking it intravenously in pure, ethanol form, ethyl alcohol can make you more prone to spilling your secrets.

But as you probably know, it doesn't make you incapable of a little white lie every now and again.

Do truth serums work?

As Washington Post reporter David Brown wrote in 2006: "The answer appears to be: No. There is no pharmaceutical compound today whose proven effect is the consistent or predictable enhancement of truth-telling."

Despite the fact that truth serum's magical capabilities seem to be mostly fictional, US courts have in special cases allowed their use.

One example was with with suspect in the 2012 Aurora, Colorado, theatre shooting — a judge allowed the use of sodium pentothal to determine if his claims of insanity were real.

Just because no truth-inducing drug exists today, doesn't mean there couldn't be one in the future, according to Mark Wheelis, a professor and expert on the history of biological warfare and biological weapons control at the University of California Davis.

"There is a large number of neural circuits that we are on the verge of being able to manipulate — things that govern states like fear, anxiety, terror and depression," Davis told the Washington Post in 2006. "We don't have recipes yet to control them, but the potential is clearly foreseeable. It would absolutely astonish me if we didn't identify a range of pharmaceuticals that would be of great utility to interrogators."

A truth serum experiment

To find out if truth serum works, TV journalist Michael Mosley experienced it for himself in 2013.

To investigate sodium thiopental, one of the more popular truth serum drugs, Mosley took two different doses of it. After administering the first dose, a doctor asked Mosley what he did for a living and — through fits of giggles — Mosley managed to lie and say he was a world-famous heart surgeon.

In less than a minute after the drug was administered, Mosley was all laughs from the light-headed, tipsy feeling he said he experienced from the drug. He said the feeling was akin to drinking a glass of champagne.

After the second, larger dose of sodium thiopental, Mosley experienced something he was not expecting. When the doctor asked him what he did for a living he immediately responded:

"I'm a television producer. Well, executive producer, well, presenter, some, mix of the three of them."

Mosley explained later that when asked the question, it didn't even occur to him to lie, so he didn't.

But is this evidence that truth serum works? Not exactly.

One of the biggest problems with using truth serum for interrogation, is the warm, friendly feeling it gives the subject toward their interrogator. Combined with a state of severe disorientation, this can lead a subject to tell their interrogator what they think the interrogator wants to hear, which could be true or not.

This is partly why any statement made under the influence of a truth serum drug is inadmissible in US courts and has been for 60 years. In 1963, the US Supreme court ruled that confession statements made under the influence of truth serum drugs were "unconstitutionally coerced" threatening citizens' rights under the Fifth Amendment and were therefore inadmissible.

So, when it comes to drugs that alter your state of mind, disorient you, and loosen your tongue, believe what you will about their abilities to enhance truth telling. The evidence shows that statements revealed under the influence have a chance of being more complacent or outright false than true.

Moreover, truth serums are mostly "useless", Esther Inglis-Arkell wrote for io9, "not because no one could get information, but because everyone could get too much." And sifting through the statements trying to pull out the ones that are true versus complacent is, frankly, impossible.

But researchers continue to look for something more reliable.

The future of truth-telling drugs

As we learn more about the brain and discover new drugs we could be on the verge of a new type of truth-telling and trust-enhancing drug.

One of the more recent drugs examined for its truth-telling affects is oxytocin, known to women in labor as Pitocin.

In 2005, two researchers at the University of Zurich examined the trust-promoting effects of the drug by studying 128 college students, some of whom were given a snort of oxytocin while the others received a placebo.

They were asked to play an investing game in which they had to trust a stranger to give them back a portion of their winnings. The students given oxytocin were more trusting and transferred more money, on average. More importantly, 45% of the students on oxytocin transferred all of their money, showing maximal trust, twice as many as students who took the placebo.

These new drugs that increase trust could be a next-level advancement in truth serums — they would actually encourage truth-telling instead of just making the teller say whatever makes their questioner happy.

Don't worry too much — your secrets are safe for now.