Chemistry of crime: The real-life science behind Agatha Christie’s poison plots

Crime novelist Agatha Christie was famous for her use of poison to kill off unfortunate victims in her books, with the poison itself often becoming a central part of the story. However, her choice of deadly substance was far from random, as Christie’s career as a chemist during both world wars meant she knew how to use the characteristics of each poison to provide vital clues for the discovery of the murderer.

To mark the 125th anniversary of the Queen of Crime’s birth, chemist and author Dr Kathryn Harkup has provided a fascinating look at the poisons used in the Christie’s work with her new book A is For Arsenic. The book covers 14 poisons used in 14 of the famous novelist’s books, explaining why she chose them and the science of how they effect the body.

Here, Dr Harkup reveals the gruesome truth behind two of the poisons Agatha Christie used to dispatch the victims in her novels…

We’ve all seen the moment on TV when the wealthy relative or despised ex-lover takes a mouthful of poisoned food, twitches a bit and collapses on the floor dead. Fortunately or unfortunately TV often edits the horrible reality of poisoning. Crime writers on the other hand often go to extraordinary lengths to maintain scientific accuracy in their books and, though some things may be sped up for the benefit of a pacey plot, the details are often reliable.

Agatha Christie is noted for her accurate descriptions of poisons in her plots. Few crime writers can claim the dubious honour of being mentioned in a criminal trial. The toxicologist involved in the trial of notorious poisoner Graham Young said Christie’s description of thallium poisoning in her novel The Pale Horse was, “the most reliable outside of a specialist textbook.” But what sets Christie apart from other crime writers is how often she used poison in her plots and the impressive variety of compounds she employed to dispatch her victims. Christie’s toxic tally tops 30 different compounds from the well-known arsenic, strychnine and cyanide, to the more obscure physostigmine, gelsemine and aconitine.

Horrible hemlock

Conium maculatum

Take the example of hemlock, which Christie used to great effect in her novel Five Little Pigs. She doesn’t just add chopped up bits of the plant to a salad or boil it up to make soup, Christie uses a toxic component of hemlock, coniine, extracted from the plant Conium maculatum. She describes the paralysing effects of coniine on the muscles and mentions how it was once used to treat asthma. In the novel, the coniine is administered in a glass of beer. The victim complains of the bitter taste and later he is seen to stagger but this is thought to be because he is drunk rather than because the poison is making its slow progress through his body. Fortunately Poirot is asked to investigate and realises the true significance of these observations.

Coniine kills because of its interaction with the nervous system. Receptors that normally bind acetylcholine, an interaction that causes a nerve to fire, are blocked. When the nerve cannot receive signals it cannot pass on the message to the muscles to move. A slow, creeping, paralysis begins with the feet and works its way up the body until the muscles used for breathing are paralysed. This can take hours and the victim is awake and lucid until the very last moments.

Deadly digitalis


In Appointment with Death Christie accurately describes how an overdose of digitalis kills by paralysing the heart. Digitalis, a mixture of related compounds isolated from species of Digitalis plants, commonly known as foxgloves, have been used to treat heart conditions since the eighteenth century. Dr William Withering was the first to investigate the medicinal properties of foxgloves and showed that doses of the dried leaves administered by mouth increased the intensity of the heart’s contractions and slowed the heartbeat. He also noted the toxicity of the plant and made detailed descriptions of the effects of higher doses. Much later, the different components of digitalis were isolated and investigated. Today the most potent compound with foxgloves, digoxin is used to treat atrial fibrillation, the rapid and uncoordinated beating of the atria section of the heart. The drug is hugely successful.

Digoxin interacts with enzymes in the heart that are involved in nerve signals. The effect is twofold, the electrical signals that move through the nerves to coordinate the heartbeat are slowed and the heart cells contract more intensely resulting in more efficient pumping of the blood around the body. Too much digoxin blocks the signals altogether and the heart stops, mimicking the symptoms of the disease it is prescribed to treat. This was extremely useful to the murderer in Appointment with Death who had hoped that the sudden death of the victim would be blamed on her weak heart. However, the murderer did not know that a world renowned Belgian detective was holidaying nearby. When Poirot is called on to take a look at the case he quickly realises it is a case of murder and justice prevails.




There is a lot more to Agatha Christie novels than first meets the eye. The scientific detail is often used to reveal vital clues to the murderer. However, readers with a scientific background have no advantage over anyone else and it is still a surprise when the culprit is finally revealed. Christie was also careful to explain all the necessary details to her readers in clear and accessible language for those without any scientific knowledge. Reading Agatha Christie with particular attention to the science has given me a whole new appreciation of the accuracy and creativity Christie displayed in her plots.

A is for Arsenic by Kathryn Harkup, published by Bloomsbury Publishing, is now available in hardback for £16.99. For a chance to win a copy of the book, plus six other titles from the popular science imprint Sigma, you can test your knowledge of Christie’s poisons with this Poison Pen quiz

A is for Arsenic by Kathryn Harkup
A is for Arsenic by Kathryn Harkup

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