Murder, she measured: The impressive science behind Agatha Christie’s poisons

Dame Agatha Christie stacked up quite the body count during her long and esteemed career. The “Duchess of Death” wrote 66 detective novels, 14 short story collections, and 20 plays. Across those tales, her many victims were shot, bludgeoned, stabbed, electrocuted, strangled, run over, drowned, axed, and pushed off all manner of precipitous edges — from stairs to cliff sides — to let gravity sort out the messier details.

Christie has probably envisioned more creative ways to murder someone than Hannibal Lecter, Dexter Morgan, and Freddy Krueger combined, and readers have enjoyed solving her puzzle-box stories for a century — making her one of, if not the bestselling author of all time.

I’m one such reader. I’ve adored Christie’s stories since I picked up a well-worn Pocket Books edition of Murder on the Orient Express in a used bookstore my freshman year. The book is considerably older than I am yet retains a treasured perch on my bookshelf — that subtle fragrance of vanilla aged books acquire growing stronger with each reread. Value for money, it is easily the best $2 I’ve ever spent, and it kindled an enduring fondness for Christie’s characters.

But it wasn’t until I spoke with Kathryn Harkup, a chemist turned science writer, that I began to appreciate a side of Christie that I hadn’t noticed before.

“She’s a brilliant science communicator. She puts across all the science you need to know in her stories to be able to figure it out,” Harkup tells me during our conversation. “You (probably) won’t because she’s very good at disguising who’d done it. But all the science is there, and you never feel that you’re sitting through a chemistry lesson.”

Specifically, she is referring to when Christie plays the literary poisoner, a role that has delivered some of the author’s most beguiling and devious mysteries. 

Harkup has dedicated two books to Christie’s poisons, the bestselling A Is for Arsenic (2017) and its sequel, V Is for Venom (2025). In each, she explores a dozen of the author’s chemical killers: their composition and toxicology, their history and traditional uses, how Christie employs them, and how their antidotes work (if they have one). She even dives into some disturbing real-life cases.

The books are part history, part scientific explainer, and part literary criticism that aren’t only fun, informative reads in their own right. They also heighten your admiration of Christie as a writer.

Christie’s goal was always to keep the readers guessing by delivering the deadly blow in an unexpected yet ultimately credible manner. When it came to her poisonous plots, she never drew attention to the science so as not to distract from the mystery. But in doing so, Harkup reveals, she leaves all the clues necessary for readers to begin learning about these noxious chemicals.

Death by chocolate (but mostly poison)

When most people think of science communication in stories, their minds turn to science fiction. Makes sense — it’s right there in the name. This reputation is further cemented by the fact that many of the genre’s most respected writers either worked as scientists or held positions in scientific organizations. Think Isaac Asimov, Arthur C. Clarke, and Gregory Benford.

Like those authors, Christie brings a scientific background to bear in her stories, too. During the First World War, before becoming a writer, she worked as a pharmacist’s assistant at her local hospital. Before securing the job, she had to pass the exams and undergo strenuous training to handle the many chemicals stocked on the pharmacy’s shelves.

As Harkup points out, back in Christie’s day drugs didn’t come ready in pre-packaged pills. Pharmacists and their assistants were more like chemical cooks than dispensers. They measured, weighed, mixed, and packaged all manner of medical tinctures and creams in-house. They even added flavors and colorings to make bitter or unappealing concoctions palatable.

Medicinal mishaps could prove deadly, and further complicating matters, prescriptions weren’t yet standardized. Doctors had their preferences for treatment and dosage, and pharmacists had to be careful. A mistake could deliver an unhealthy dose or combine drugs that react poorly together. In V Is for Venom, Harkup even shares a story where Christie saved a patient from a dangerous situation by noticing a mistake in the pharmacist’s calculations.

“Christie understood the difference between the therapeutic and lethal dose as well as appreciating the dangers of not doing her job properly,” Harkup writes. She also notes how the job introduced Christie to the many “quirks and little stories” surrounding these chemicals, showing her how people used them or how they might realistically find their way into someone’s home.

This experience and knowledge proved a boon to a future writer of detective stories.

Consider her short story “The Chocolate Box” (1923). Its victim, one Monsieur Paul Déroulard, dies from sudden heart failure. Those present note how his face flushed bright red before he collapsed to the floor. The cause was an overdose of nitroglycerin secreted into his favorite chocolates.

That may seem an odd, almost arbitrary choice. Nitroglycerin is famously the explosive ingredient in dynamite. Deadly, sure. But actually eating the stuff unawares? Hardly plausible, right?

It turns out that a small dose of nitroglycerin is also a reliable treatment for anginal chest pain because it dilates and relaxes blood vessels. A surplus, however, can drop a person’s blood pressure to fatal levels. Even so, it is generally considered safe and is still administered today in patches, sprays, and extended-release capsules. Early-20th-century pharmacology took a different route, though: chocolate. 

The nitrate’s naturally sweet taste meant it paired well with cocoa. In fact, it is “the only medicine to have been officially prepared in chocolate tablets,” Harkup writes, and a book Christie likely studied for her exams, The Art of Dispensing, contains several recipes for nitroglycerin tablets. Thanks to this confectionary camouflage, the poor Déroulard never tasted the deadly additive.

In the story, this information — from the symptoms to the drug’s preparation to how they came to be in the house — is true to life. However, it isn’t presented as a lesson by some all-knowing character. It is revealed naturally as clues Poirot gathers by speaking with characters and making observations. The result feels far more engaging and dynamic than the vicarious lectures so many other stories, detective and science fiction, lean on.

Christie even one-ups her true-crime counterparts with this one. As Harkup points out, plenty of attempted assassinations have gone the death-by-poisoned-sweets route. In one famous case, someone tried to lace a walnut whip with strychnine (read: rat poison). The intended victim never ingested a lethal dose though. The reason: The foul taste led them to put down the chocolate.

“It was surprising where [Christie] found her inspiration,” Harkup says. “She was certainly a reader of true crime and fictional crime, but her pharmaceutical knowledge meant she could go to places where even criminals hadn’t thought of going.”

Murder is queasy

Another reason Christie’s poisons communicate their science so well isn’t just the nature of the tools. It’s also the nature of the crimes. As Harkup and I discuss, poison isn’t a heat-of-the-moment weapon. You don’t slip arsenic into someone’s tea because they cut in front of you at the bank.

“You really have to think about it,” she says. “You have to acquire the stuff. You have to figure out where you’re going to put it. Then you have to sort out your alibi and make sure the person doesn’t survive because some poisons are quite survivable.”

Such premeditation must be built into the story and revealed slowly through the detective’s investigation. This not only makes the clues satisfying to piece together but also allows Christie to make her poisons “characters themselves within the story” — complete with backgrounds and a kind of personality.

“The way the poisons will behave, you can sort of make predictions about it, and you can tell the characteristics of who might use them,” Harkup says.

A great example comes from the novel Murder Is Easy (1939). One of its many victims — the title is apt — is killed using oxalic acid. Oxalic acid isn’t uncommon for a poison. In fact, you probably have some in your home right now. 

A low-grade corrosive, it’s found in cleaners and stain removers. It also occurs naturally in many foods. For instance, rhubarb stems are perfectly safe to eat if cooked, but the leaves should be tossed out because they store a potentially unsafe concentration of the stuff. Oxalic acid is so common that the trick isn’t acquiring the stuff; it’s finding a way to administer the poison that makes for a surprising yet satisfying mystery. And Christie doesn’t disappoint.

In Murder Is Easy, the killer swaps a bottle of cough syrup for hat paint, the latter of which contains a potent amount of oxalic acid. Back in the day, hat paint was used to change one’s on-the-town look without the expense of a new hat; however, it fell out of fashion decades before the story takes place. It’s also a poor choice of poison because it is easily treated. Had the killer not delayed medical aid by haphazardly jamming the door with a pair of pincers, the victim may well have survived.

Using the poison’s history and its characteristics, Christie subtly presents several important clues to who the killer is: their age, gender, social status, and even some elements of their personality. Slot those clues in with the others sprinkled throughout the story, and a sleuthy reader should know who the killer is well before the reveal.

Pick your poison

I asked Harkup what her favorite Christie poison is, and she chose Christie’s use of hemlock in Five Little Pigs* (1942).

“It is brilliantly woven into the plot to the point where you need to read it twice to appreciate how good it is,” Harkup says. “When you go back, you’re like, ‘Oh God, yeah. I should have spotted that. That was damn obvious if I had paid attention to that particular bit.’ Her creativity is astonishing in that respect.” 

She adds that Christie’s use of thallium poisoning in The Pale Horse (1961) is so accurate that the book is credited with saving two lives thanks to readers recognizing the symptoms from its description.

Of course, I had to know if Christie made any serious blunders or stretched the truth to the breaking point, and Harkup admits that Christie will sometimes push plausibility. For instance, she often has to concoct situations where medical aid is delayed, like in Murder Is Easy. Otherwise many of her stories would be shelved under attempted murder mysteries. She’ll also sometimes lower the deadly dose to something more manageable, like in “The Chocolate Box,” or speed up poisonings so readers don’t have to read about “hours of vomiting and diarrhea and convulsions” (which I definitely appreciate).

Even so, Harkup adds, Christie doesn’t fall back on the expedience of lazy poisoning clichés like many modern crime dramas —  where someone takes one sip of wine, coughs twice, and falls down stone dead. Nor does she fabricate phony poisons like Arthur Conan Doyle did for Sherlock Holmes or Shakespeare in Hamlet.

Not that that’s a problem. “It only matters for a writer to be accurate within the world they have constructed. It’s an invented world, so they can do whatever they want as long as it obeys the rules of that world.” Even so, Harkup adds, “As a chemist, I appreciate it when people like Christie go to so much trouble to get things right.”

Death comes as the end

As mentioned, Harkup also chronicles several true crimes featuring Christie’s poisons. “I was unfortunately blessed with an abundance of examples of the worst of humanity,” she says. 

For instance, in 1901 a young woman was found dead with nitroglycerin residue in her stomach. Her suitor was brought to trial, where he was found to have provided the nitroglycerin under the false belief it was an abortifacient. However, the exact cause of death remains a mystery since the amount of nitroglycerin likely wasn’t enough to kill her. Alcohol, other substances, and perhaps even the morphine the doctors administered while treating her convulsions may have contributed.

Another incident Harkup recounts involves oxalic acid killing two people in 2015. The poison was found in Hokkaido tea, and given the amount, officials strongly suspected it was added deliberately. However, without concrete evidence or a strong motive pointing to a suspect, no one has been charged. The case remains unsolved.

“The poisons have changed. The manner in which they’re administered has changed, but it still happens,” Harkup says.

What I found interesting about these true crimes wasn’t just the history but how they left me feeling. After reading them, a cold discomfort settled over my afternoon like a fog. That’s understandable given what these tales reveal about people, their inhibitions, and the random cruelty that can bring life to a sudden, unfair end.

But why do I find Christie’s stories so appealing then? Why, despite the murder, greed, and the worst of humanity on display, do these stories feel so warmly comfortable?

I asked Harkup this question, and she answers that Christie’s fiction helps readers slake certain psychological thirsts that real life often fails to satisfy. The first is our desire for order and justice in a chaotic world. 

“We want things to be fixed, sorted, and solved. I think there is a very basic need for us to have the bad guy punished and the good guys come out, if not victorious, then at least acknowledged in their rightness,” Harkup answers.

And what about the poisons themselves? As Harkup reminds me, many people are drawn to the risker side of science — the things that blow up, go boom, or move at supersonic speeds. Chemistry, she jokes, is a professional home for a bunch of “controlled pyromaniacs.” By making these dangerous chemicals the center of a fascinating puzzle, she combines that scientific desire for a solution and a safe amount of danger with a great story.

“It’s that extra layer of nastiness that fascinates us,” Harkup says. “[Stories are] a safe way of finding out about something that is so unknowable since the vast majority of us — I would hope — will never kill another human. It’s a safe trip into the darkest bits of the human psyche.”

* Author’s note: I won’t highlight any important clues here like I did with Murder Is Easy. If you haven’t, I highly recommend giving Five Little Pigs a read yourself.