Instant Genius - The science behind Agatha Christie’s poisons

Episode Date: June 12, 2025

Humans are enthralled by stories about murder. And the mysteries we tell these days – like in the Glass Onion films or TV shows like The Residence – often follow a blueprint set by the iconic Agatha Christie. It turns out, Christie knew a whole lot about science. In this episode, we speak to Dr Kathryn Harkup, a chemist who writes about the science behind famous works of literature. Her newest book, V is for Venom: Agatha Christie’s Chemicals of Death, is her sequel to A is for Arsenic: The Poisons of Agatha Christie, exploring Christie’s expert use of dark chemistry. She tells us about some of the poisons Christie used in her books – the brutal, the medicinal and the obscure – revealing Christie’s extensive chemical knowledge. But be warned: this conversation gets quite dark. Listener discretion is advised. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Hello, welcome to the Instant Genius podcast. I'm Hattie Wilmoth, a trends editor at BBC Science Focus. Humans are enthralled by stories about murder. And the mysteries we tell these days, like in the Glass Onion films or TV shows like the residents, often follow a blueprint set by the iconic Agatha Christie. And it turns out, Christie knew a whole lot about science. In this episode, we speak to Dr. Catherine Harcott, a chemist who writes about the science behind famous works of literature. Her newest book, B is for Venom, Agatha Christie's Chemicals of Death, is her sequel to A is for arsenic, the poisons of Agatha Christie, exploring Christi's expert use of dark chemistry.

She tells us about some of the poisons Christy used in her books, the brutal, the medicinal and the obscure, revealing Christy's extensive chemical knowledge. But be warned, this conversation gets quite dark. If you're not up for learning about the gruesome ways poisons can kill people, this episode might not be for you. Hi, Catherine, welcome to the podcast. Thanks for being here. Thank you for inviting me. We're going to be talking a little bit about your book, V is for Venom, Agatha Christie's Chemicals of Death.

Ooh, a bit scary, a bit of a scary title. I wanted to ask you, first of all, as a chemist, What do you find so intriguing about poisons? I find it absolutely fascinating that something in such a tiny quantity can bring about the end of something as big and as resilient as a human being. Because the sort of quantities that a lot of poisons, like the big famous poisons that you've probably heard of, arsenic cyanide, things like that, it is tiny, tiny amounts that can end your life. And I found it intriguing as to how it could achieve that. What must it be doing inside the body to bring about this catastrophic result? Because all of them do it in slightly different ways.

There is no kind of like, if you take a poison, this will happen. There is a diverse range of symptoms, ways that people can intervene, and also a diversity of outcomes. So fascinating stuff. And that was something I was surprised about when reading your book was just that every single poison works differently and is different and has different effects. It surprised me as well because I was slightly worried. I knew that there were three different acids that I was going to be writing about.

And acids have very similar effects in terms of corrosive properties. But in actual fact, they all have different ways of interacting with the body, despite the fact that they are all acid. So yeah, it was fascinating to me as well. And the whole book is hinged on Agatha Christie and the poisons that she used in her books. What's so special to you about Agatha Christie? Oh, she's a great writer. She is very much a comfort read for me, which seems grim because she's always bumping off lots of characters. And she describes a lot of death.

But I enjoy her writing, but I also, as a chemist, really, really appreciate the diversions. and the accuracy of the poisons that she used in her stories. Because it is, I don't know of any other author who achieved that kind of level. And I appreciate she wrote a lot more books, so she had more opportunity. But she needn't have done, she could have stuck with the, you know, the tried and tested arsenic, but she didn't. You've read so many Agatha Christie books. Do you have a favorite poison that she used to kill someone in one of those stories? I do have a favourite, and it is purely based on my chemistry background, because ask any chemist, they will all have a favourite element in the periodic table.

And my favourite element is phosphorus. And Agatha Christie, Godlover, she used phosphorus as a poison in just one novel. And it's glorious. It celebrates all of the gruesome things, all of the gothic traditions that drew me to phosphorus as a fantastic poison. So, yeah, I really appreciate that she went to town with her knowledge of phosphorus in. I'll tell you, it's dumb witness if you want to read it. So what is phosphorus? How does it work?

How does it kill people? Phosphorus, it's an element. It comes in lots of different forms. The form that kills people most effectively is white phosphorus, particularly if you swallow it, although there are hazards of inhaling it and being exposed to it in other forms, as a lot of match workers knew in the 19th century, because phosphorus was used on the match head to cause that little spark of ignition

when you rubbed against a surface. So phosphorus, it's very flammable, but that's not how it kills you. What it actually does is if it gets inside you, it interacts with your liver. The details are a bit sketchy, even today, but the result is your liver stops working. The antidote to phosphorus poisoning is a liver,

liver transplant, and those are not easy to come by. There's no other way to cure yourself if you've been poisoned with phosphorus than having a liver transplant. If it's got into you, into your body proper, and I'm not just talking about your stomach, so if they get to you early, they will remove it from your stomach so that it can't be absorbed into the body. But once it's in your body, yeah, you've got to maintain liver function, and quite often, yes, you need a liver transplant.

So does phosphorus have any useful properties? It does have useful properties. I mean, it's still involved in matches. It's just that it's not white phosphorus anymore, because that's the really nasty stuff. It's a red phosphorus that's used, which is not toxic, and it's the red stripe that's on the side of the box. I mean, don't lick the side of match boxes,

because it's still not good for you, but it's not the phosphorus that's going to do any harm. So it has uses in that. It also has very sort of niche uses in chemistry that aren't particularly every day. It is something that you encounter every day in the form of phosphate because you eat it in your food. It makes up your DNA. It strengthens your bones. Phosphorus is in you everywhere. There's almost a kilogram of phosphorus in your body, but it's in the form of phosphate,

which is perfectly safe and frankly essential to health. so many of these poisons are in themselves in small amounts or in different forms useful to us, often have been used as medicines. Do you have any favourite examples of those kind of poisons? Oh, there are lots of examples. I was surprised by things like formic acid that was originally distilled from ants. It's in food as a preservative and it's fine. You can eat it in small doses. It's perfectly safe. It's when it's in a concentrated form and you have it in one go that things can go very, very horribly wrong, very, very quickly. So that's the truth of all poisons, everything, literally everything is a poison if you have too much. It's just that that quantity that constitutes too much varies enormously depending on the compound. So for formic acid, it might be, you know, less than a shot glass.

if it's in a fairly concentrated form, but in food, over time, you will eat far more than that, and you'll be perfectly fine. One example from this book might be something like Strafanthin, which is an arrowed poison. So this was something that was found in East Africa. It was used by the locals to tip their arrows so that they could hunt large game, and it would again affect the heart and it would bring down these large animals with minimal danger to the hunters themselves. So extremely useful. And because of this interaction with the heart, people realize that with an appropriate dose, you could actually correct a defect in the heart

and it might actually be useful as a medicine. So there's a brief vogue for having strifanthin as a medicine. It never really caught on because there was another very well-known heart drug that had been in use for centuries and it also you could get it in Europe. You didn't have to go to Africa to source it. And that's digitalis or foxgloves. So foxgloves were the heart medicine of choice. But both foxglove and strifanthin are extraordinarily toxic. You really do need tiny, tiny amounts for a therapeutic benefit. And people who take these drugs, lots of people take digitalis or digotoxin, digoxin, even today. And it saves thousands and thousands of lives every year. It is a brilliant heart medicine. But these people are very carefully monitored

to make sure they're not taking too much. Because taking too much, again, something as important as the heart that you need to keep beating from one moment to the next, you really do have to look after it. And so, yeah, they make sure they get the dosage right. And if things look as though they're starting to go wrong, there are alternative drugs that you might be switched to. And there are all sorts of precautions in place now that just didn't exist in the past. One of the things that found surprising was just how many examples you mentioned in your book of substances that had previously been used as mental health medicines? In the past, certainly, there was an element of self-medication with things like opium, lardinem, which was a mixture of opium and alcohol. So you were either

going to get high from the opium or you were going to get drunk from the alcohol. And the combination of the two would work together to intensify the reactions in your body. So there was a lot of that. And it was astonishing to me that it wasn't until the the 1950s that we had anything that was really useful against certain forms of mental illness in terms of drugs. We have barbiturates, which were again, they are used to sedate people. And yes, they will make you feel better. And they're a bit of a pick me up and they will make you feel more assertive, which is probably why a lot of celebrities ended up taking them just to, you know, get them through performances and things like that. And they've became.

addicted. But they weren't really good as mental health drugs. They were sedatives. And it wasn't until things, like I say, in the 1950s that things changed. And which of those that you mentioned, I think barbiturates was one that you mentioned that was also used as a poison in Agatha Christie's novels? Was that right? Yeah, there's a lot of barbiturates floating around in Agathar Christie's world. They turn up all over the place, mostly people who are, again, quite strait. and they need something to sleep, or it is associated with the rich and famous. So there's quite a lot of drugs floating around, particularly actors and actresses, seem to have quite a bit of it stashed away.

And it was quite common, even though you had to have a prescription to get barbiturates, because the dangers of barbitur overdose were recognised very early on. You needed a prescription to get hold of it, but once you've got your prescription, you could go back with that same bit of paper and get refills. So it was still quite easy to get hold of dangerous amounts. And like with a lot of those drugs, as I mentioned before with opium, once you mix them with alcohol, all of the effects are intensified and it becomes positively dangerous.

So it's not just one drug on its own necessarily interacting with the body, but it's how other compounds that might have been swallowed alongside it interact as well or work with each other to make things better or worse. And chemically, why does alcohol have that effect? It's simply that it interacts with the same receptors as barbiturates and a lot of those other drugs, but a different part of the same receptor. So these receptors are giving a kind of calming influence to the brain. They're relaxing you.

And obviously the danger is if it relaxes you to the point that you stop breathing, that is fatal. So if you have two of these drugs working together, they intensify their effects, so it's greater than the sum of their parts. So two or three drugs operating on the same receptor makes things a whole heap worse. And chemically, how do barbiturates work? So barbiturates interact with gabber receptors, they're called, and these are receptors that release the compound that basically calms the activity of a nerve. So nerves in your brain primarily, but it can also interact with other nerves in the body. So if you've got an over-excitable bit of your brain that's firing a bit too readily, if you can activate GABA receptors, it will calm that down. And you should feel better

if that is the appropriate response that you need for that particular condition. It encourages the release of a natural compound within your brain that calms those nerves in your brain down. This episode is brought to you by Netflix. Most valuable promotions in Netflix are hosting a blockbuster triple headliner Saturday, May 16th. Rhonda Rousey returns to face fellow woman's MMA pioneer Gina Carrano in the main event. Plus co-main's Nate Diaz versus Mike Perry and the best heavy weight in the world, Francis Ngano versus Felipe Lins. Watch Rhonda Rousey versus Gina Carano.

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name audio creates systems that deliver exceptional sound and unforgettable listening experiences at home. Try it for yourself at a focal powered by name boutique. Visit Focal Powered by Name Boutique. by name.com for more information. So we've sort of touched on poisons that had medicinal uses. Did you have any more examples of those that you wanted to give? I guess nitroglycerin is a fun one. I'm not sure that many people realise that nitroglycerin,

which is the explosive part of dynamite, is also a very valuable medicine still today. It is still used because it breaks down in the body a bit like it breaks down in a chemical explosion, but at a slower pace that is much more acceptable within the body. And it breaks down to release nitrous oxide. And this nitrous oxide is, again, it's naturally present in our body. We're just encouraging more of it.

And nitrous oxide is a chemical signal to cause veins and arteries to dilate and allow more blood to flow through them. So this is very good if you have a block. artery leading to your heart, for example, causing angina or chest pain, etc. So you dissolve a tablet that's got a little bit of nitroglycerin in it under your tongue. It gets absorbed into the body. Your veins and arteries dilate and you get more blood flowing to your heart and you feel better in seconds. It's fantastic and it's a huge benefit medicinally. Amazing. And then how does it kill people? So if you dilate the veins and arteries too far, then you get a catastrophic drop in blood pressure.

And if you don't have enough blood, you need some blood pressure so that the heart's got something to push against and pump it around your body. And if your blood pressure drops too far, then obviously that can't happen and it can be very rapidly disastrous. And then also, okay, so on the flip side, from the medicinal side of things, there are all those sort of point, sort of poisons that you mentioned in your book that are actually just diseases, like bacteria. I wouldn't think bacteria is a poison. Can you talk to me about that? It is a strange concept, I suppose, that these tiny little single-celled organisms, again, can cause such massive disruption to something as big as a human.

What is it? What are they doing to the body? Because we outsize them enormously. And yet they cause us such problems. And it is, again, it's down to chemistry. and it's down to poisons. So they are releasing chemicals as part of their normal life cycle. It might be part of their growth, producing more of themselves, you know, just normal day-to-day activities. It just so happens that those chemicals don't interact with our body very well and they

cause us problems. In other cases, the bacteria releases these chemicals specifically to maybe destroy areas around them so that they have space to grow into. So it is part of their sort of colonization of an area. So there can be all sorts of chemicals that are released. And even one bacteria can release several different chemicals that might cause us problems. And some are nastier than others. Some we can deal with, some we can't. I would say that the vast majority of bacteria are beneficial or simply we're indifferent to them. Our bodies contain, more cells of bacteria than cells of us, which if you think about it too hard can get slightly worrying.

But we are part of bacteria are in us. They are doing incredible work for us. We wouldn't be alive without them. It's just that some bacteria we definitely cannot live with. And can you tell me about some of the Agatha Christie stories where bacteria was used as a weapon? I think one of the most intriguing ones for me was in a book called Cards on the Table. So, slight spoilers, there is a murderer who infects a shaving brush with anthrax, which is a bacteria. So he manages to get this anthrax onto a shaving brush so that when his victim shaves,

he introduces the bacteria into cuts on his face, and it leads to a fatality. Now, this guy was particularly unlucky. Anthrax is nasty. There's no two ways about it. It's awful. But skin infections tend to be the least dangerous. It's about, I think it's about 20% fatalities untreated, which is still horrendously high. But it gets worse if the anthrax gets into your lungs or if it gets into your blood.

And then you've got less than a 50-50 chance of surviving that. It just gets worse and worse and worse. So some of these poisons get quite niche, and many of us haven't heard of them. But what about the everyday poisons that we might walk past on the street and not even realize their poisonous? I'm thinking things like foxgloves, yew trees, apple pips. Do you have any favourite tales of how these work? I think the yew trees is a really nice example. I think definitely in the UK we tend to associate them with death.

quite readily, but mostly because they're in churchyards, because they've been there for hundreds, if not thousands of years, and places of worship have kind of grown up around them, as well as places of burial. So they are associated with death, but there's a second reason that they're associated with death, and that's because yew trees are toxic, like every single part of them is not good for you, apart from the red berries. They're not technically berries. I think they're called Arals because they don't cover the seed inside. Those bits, in theory, you could eat if you're very careful not to chew the seed. You make sure it's ripe and you don't get any other bit of the yew tree. You're probably okay. I wouldn't chance it, basically. And I don't think they

taste very nice anyway. So just write that off completely. I think it's interesting that a tree would have that much toxin throughout. It's not just in the leaves or the roots, it's everywhere in the tree. So these trees used to be highly prized for the wood to make bows and arrows. So all of those English long bows that won Agincourt and made a serious mark in wars throughout history, they were made with U. And Shakespeare talks about, was it double deadly U? And referring to one, it's bow and arrows, two, it's toxic. So if you're hit with a U arrow, you've also got a problem because of the toxicity of it. So the toxin in U-trees, it interacts with your heart and it disrupts the normal function of your heart, which

obviously has fairly rapid consequences. There's usually unconsciousness and collapse. There was a lady in Canada who had prepared some kind of meal with you leaves, the little sharp needle-like leaves. She'd probably picked them by mistake or thought that they were edible. They absolutely are not. And she's in the middle of a video call with her friend and she collapses. And unfortunately, because her friend was miles away, she wasn't able to get rapid care to this lady. And unfortunately, she died. So it is, be careful what you eat. Make sure, if you're one of these people that likes to go out and pick, you know, wild food, while growing strawberries, berries, whatever, make sure you know what it is. If you don't recognize it from your dinner plate,

don't eat it. But they are beautiful, beautiful trees and they are spectacular. Like there was a tree in the village where I grew up, and apparently it was mentioned in the Doomsday book. I never actually looked to check. But it was all hollowed out. And this thing that's clearly hundreds and hundreds and hundreds of years old, still living, still putting out fresh leaves. And it just, yeah, they're spectacular trees. So chemically, how do you trees affect the heart? Okay, so taxine interacts with specific channels within heart cells. So the cells within the heart beat themselves, and it's this coordinated effect of beating cells overall that makes the walls of your heart move and squeeze the blood around your body.

So all of this has to be coordinated. So to coordinate this, you have the movement of things like sodium and calcium in and out of cells that encourage the contraction or the relaxation of these cells. What taxine does, which is the nasty part of you, it disrupts the flow of the sodium and the calcium in and out of these cells. So you can end up with some cells beating out of time with other cells. And once you lose this coordination over the whole heart, you can't effectively pump blood around your body.

So depending on how severe this interaction is, depends on how severe the consequences might be. And if you can get to someone in time, there are lots of drugs that can help control the normal rhythm of the heart, and there are interventions that can be made. It's just making sure you get to someone. Absolutely terrifying. The poison which, without doubt in this book, I found the most gruesome was hydrochloric acid. which I think surprised me because it's stomach acid, right? We have it in our bodies anyway. But can you tell me why hydrochloric acid can be so dangerous and how?

I think it's fantastic that our bodies have this little compartment that contains quite corrosive acid and we're fine. We just walk around with that little bag of acid inside us without any problems. And you notice those problems if you have acid reflux where that acid goes up into the esophagus, and it feels awful. You know, heartburn, all of that stuff, it's really painful. So you can appreciate the kind of damage and, well, certainly the kind of pain that might be the result of having acid attack you elsewhere. So it's all down to how the stomach is protected.

So you have special cells and you have a special kind of coating inside your stomach that protects you from the worst of the acid. And if that coating breaks down, that's when you get stomach ulcers and things like that that start to attack the wall of your stomach. And it feels awful. If you have a more concentrated form of acid, so the acid in our stomach is fairly concentrated,

but you can get more concentrated forms industrially, not necessarily over the counter. Don't try buying that stuff. For industrial purposes, you need quite concentrated forms. And certainly in the past, the regulations about buying that sort of stuff were much more free and easy.

In Agatha Christie's story, she's on an archaeological dig, and they have some fairly concentrated hydrochloric acid with them that they would have diluted down as and when they needed and probably to clean their tools, their trowels, etc. But they also, as I learnt,

used to wash pottery shards with hydrochloric acid, a weak version of it, to clean off all the grit and the grime from centuries of being buried under the ground. And hydrochloric acid, because it is very corrosive, will just eat away at that dirt and hopefully not at the archaeological artifact. Talking to an archaeologist friend, they were horrified that this was done because they were just alarmed that all of this damage would go through to the the actual find itself. But they seem to have known what they were doing. I don't think it's best practice at all today, but things were different in the 1930s. All of that kind of corrosiveness, if you imagine that happening to cells outside of your stomach, it is horrible. You know,

the very structure of your tissues starts to break down, holes form, you start to leak, stuff goes where it shouldn't go and it can be a very prolonged and unpleasant death Are there any other seriously seriously dangerous poisons which you were shocked to find out about when you were researching

to write this book? I think the most obscure poison that she uses is probably the venom so the title of the book Agatha Christie really did stick to what she knew So she trained as an apothecary's assistant or a pharmacist's assistant.

She knew an awful lot about drugs and medicines, which is why they crop up so much in her books. You don't get taught about venom's in the context of pharmacy. Certainly not in Agatha Christie's day. So where she picked up the idea of using a snake venom, I don't know. She read an awful lot of crime fiction. So maybe it was the Arthur Conan Doyle, the Speckled Band story, which features spoiler a snake. So she may be, this was her twist on it. But she didn't even use a very well-known snake.

This is a snake that I had to Google, because I didn't know what it was. It's a boomslang snake, which if you live in Africa, particularly southern regions of Africa, I'm sure you are well aware of. It's a tree snake. It's Afrikaans for tree snake. and these things mostly live out their lives quite happily in trees and they don't bother humans. They were for a long time, even up until the 1950s and 60s, they weren't thought to be particularly dangerous to humans, even though they absolutely and abundantly are.

It's just that it takes a bit of provocation to get a boomslang to bite you. That is not a challenge. That is simply, you know, beware. Don't poke the boomslang snake. if you spot one because it doesn't like it. So yeah, this venom, it is extraordinarily toxic. The symptoms that she describes in her use of it in death in the air, or death in the clouds, depending on US or UK publication,

the symptoms are accurate. They are slightly glossed over in places, slightly sped up in others, but basically she gets it right for an obscure poison from an obscure snake in a field that she didn't really know about. Oh yeah, what on earth was she thinking? I don't know. And how does that venom work? Why is that particular venom poisonous? So there are lots of things in snake venom typically.

There are a cocktail of all sorts of stuff going on. But the main problem with boom slang venom is that it interacts with how your blood clots. So if you get a sudden rush of it into your... veins, as this unfortunate woman does on a flight to the UK, it rapidly clots your blood. And so you can have thrombosis forming. So these blood clots can move to parts of your heart and block blood flow, and that can have very rapid consequences. If it gets introduced into the body more slowly, it can still cause clotting, but it kind of depletes your clotting resources. And it means that while your body is busy trying to make more of these clotting materials, you can't stop

bleeds. And we bleed, you know, there is damage internally a lot of the time. And our body just blocks it off and we carry on as normal. But if you've lost all of that clotting ability, you start to bleed internally and you eventually bleed to death. It takes about 24 hours. So the effects, and again, Christy appreciated this. If you had a slow introduction, her victim would have had 24.

hours to be studied, to maybe talk about who they'd seen delivering this venom. But no, she has it delivered into the bloodstream so that the death is very, very rapid and pretty believable in the way she describes it. So I think there are quite a variety of ways that poisons can hurt us. That's what I learned from your book, and it's certainly what I'm learning even more as I talk to you now. Are there any common themes or classes or categories of poisons? I suppose there are are broad types in terms of their effects on the body. So there are things that will interact with your nerves. So if your nerves start to misfire, that can have consequences on breathing, heart regulation, all sorts of basic functions within the body. Then there are poisons that

interact with the heart, some of which we've talked about today. And as we've said, you need that organ to keep going no matter what. And if you disrupt it, things can go horrible. wrong, then there are poisons that just physically break down structures. So things like hydrochloric acid, it's almost not a poison because poison to me implies a relatively narrow target, whereas hydrochloric acid, if it's concentrated enough, will just destroy everything in its path, certainly in terms of a body. So this isn't particularly specific to my mind. So is it a point, well, yeah, the consequences are it kills you, but it's in a slightly different manner. And then there's the snake poisons, which snake poisons, again, they can affect the heart,

they can affect muscle tissue, or they can affect clotting or nerves. So again, there's these broad kind of categories. In terms of the structure of the poisons themselves, I guess there are certain common themes. So, certain, and heavy metals will interact with the body in broadly similar ways, not always the same. Then there are plant poisons, which tend to be medium-sized organic compounds. Then there are other compounds that are produced by bacteria, which are big proteins, and they're quite elaborate and complex, but they have very, very specific targets. So yeah, there are kind of broad categories, but the size of a molecule and how it was produced

doesn't necessarily correlate to the effect it will have on the body and the likely outcome. Do you have any advice for anyone encountering a poison? If they've sort of ingested something which is poisonous or they're with someone that has ingested a poison? Call 999. Straight away. Don't intervene. Just call someone who knows what they're doing, okay? I know you really want to help someone, but please, please don't induce vomiting. Don't do any of that stuff that you might, God's sake, don't do the Agatha Christie standard of

given brandy. Just keep them as calm and as relaxed as possible until the ambulance arrives. And let the experts treat the person. You have time for this. The vast majority of poisons, it's not like you see on TV where someone eats a forkful of food, chokes a bit and they're dead on the carpet seconds later. You have time. And honestly, please just reassure the person that help is on its way and just dial 999. Describe symptoms, describe whatever someone might have taken to help the paramedics whoever treats them. But yeah,

don't DIY it. Thank you for listening to this episode of Instant Genius brought to you from the team behind BBC Science Focus. That was Catherine Harker. To learn more about the gruesome science behind murder mystery poisons, check out her book, V. Is For Venom, Agatha Christie's Chemicals of Death, which will be published on the 19th of June. If you liked what you just heard, then please consider subscribing to Instant Genius wherever you like to listen to podcasts.

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