Dame Agatha Mary Clarissa Christie (1890–1976), the ‘Queen of Crime’, holds the Guinness World Record as the most successful novelist of all time. She has been outsold only by the Bible and by Shakespeare (and is more widely translated than the Bard); Christie is also the author of the world’s longest-running play, The Mousetrap, and created not one but two of the best-known fictional detectives, Hercule Poirot and Miss Marple. Praise, prizes, and awards have been heaped on Christie for her work, and her books and plays are still loved by millions.

Many have attempted to divine the secret of her success. Christie always considered herself a ‘popular’ writer, and acknowledged that she did not produce great works of literature or deep insights into the human condition. Nor did she revel in gore, or try to shock her readers with gratuitous violence. Christie created many corpses in the pages of her books, but the reactions they produce are most likely to be curiosity and a smile at the prospect of clues, red herrings and brilliant deductions. She was a teller of tales, an entertainer, and a poser of seemingly insoluble puzzles.

Christie took advantage of her detailed knowledge of dangerous drugs to help develop her plots. She used poisons in the majority of her books, far more than any of her contemporaries, and with a high degree of accuracy, but she did not expect the reader to have detailed medical expertise. The symptoms and availability of drugs are succinctly described in everyday language, and somebody with a degree in toxicology or medicine has no real advantage over any other reader. An understanding of the science behind the poisons Christie used only gives a better appreciation of her cleverness and creativity in plotting.

A poisoner’s apprentice

Agatha Christie’s knowledge of poisons was certainly exceptional. Few other novelists can claim to have been read by pathologists as reference material in real poisoning cases. Several of the people who kindly read chapters of this book in its early stages have asked me ‘How did she know all of this?’ The answer is that her knowledge came from direct experience with poisons and a lifelong interest in the subject, though not in the criminal sense.

In the First World War, Christie volunteered as a nurse at her local hospital in Torquay. She enjoyed the work but when a new dispensary opened at the hospital it was suggested that she might work there. Her new role required further training,and Christie also needed to pass examinations to qualify as an apothecary’s assistant, or dispenser, which she did in 1917.

In order to prepare for the Apothecaries Hall examination Christie was tutored in practical, as well as theoretical, aspects of chemistry and pharmacy by her colleagues at the dispensary. In addition to her work and tutoring at the hospital, Agatha received private tuition from a commercial pharmacist in Torquay, a Mr P. As part of her instruction, one day Mr P. showed her the correct way to make suppositories, a tricky task that required some skill. He melted cocoa butter and added the drug, then demonstrated the precise moment to turn the suppositories out of the moulds, box them up and label them professionally as one in one hundred. However, Christie was convinced that the pharmacist had made a mistake and added a dose of one in ten to the suppositories, ten times the required dose and potentially dangerous.She surreptitiously checked his calculations and confirmed the error. Unable to confront the pharmacist with his mistake, and frightened of the consequences of dispensing the dangerous medicine, she pretended to trip and sent the suppositories crashing to the floor, where she trod on them firmly. After she had apologised profusely and cleared up the mess, a fresh batch was made, but this time at the correct dilution.

It wasn’t just Mr P.’s inattention to detail that troubled Christie. One day, he pulled a brown lump from his pocket and asked her what she thought it might be. Christie was perplexed, but Mr P. explained that it was a lump of curare, a poison originally used by hunters in South America on the tips of their arrows. Curare is a compound that is completely safe to eat but deadly if introduced directly into the bloodstream. Mr P. explained that he carried it around with him because ‘it makes me feel powerful’. Nearly fifty years later, Christie resurrected the deeply disconcerting Mr P. as the pharmacist in The Pale Horse.

By 1917, Christie had written some poems and short stories, a few of which had been published. And then, after reading The Mystery of the Yellow Room by Gaston Leroux, Christie thought she would try to write a detective novel herself, and said as much to her sister, Madge. But Madge, a more successful writer than Agatha at the time, stated that it would be very difficult, and bet her that she wouldn’t be able to do it. It was not a formal bet, but nonetheless it spurred Christie to write. It was while working as a dispenser that she found she had the time to think about the plot and her characters, and, being surrounded by poison bottles, she decided that poison would be the means of murder.

The resulting novel was The Mysterious Affair at Styles, and Christie demonstrated her detailed knowledge of strychnine throughout the book. However, she had to wait a few years and try a number of publishing houses before the novel was finally accepted in 1920. After publication Christie received her most cherished compliment when it was reviewed in the Pharmaceutical Journal and Pharmacist. ‘This novel has the rare merit of being correctly written,’ the reviewer stated. He believed the author must have had pharmaceutical training, or had called in an expert.

A criminal career

The publication of The Mysterious Affair at Styles was the start of a long and very successful career, but it was only after publishing three novels that Christie acknowledged that she might be a professional writer. She maintained her interest in poisons and drugs throughout her writing life, and only reluctantly used guns in her work—she freely admitted to knowing nothing about ballistics.The scientific details of her chosen poisons were well researched. She built up a considerable medico-legal library over the years, with the most well-thumbed book in her collection being Martindale’s Extra Pharmacopoeia.

Christie also corresponded with experts to check her facts. For example, in 1967 she wrote to a specialist asking about the impact of putting thalidomide in birthday-cake icing—how long would it take to make an impact? How many grains would be needed? However, this idea was never used in any of her stories.

In terms of poisons, Christie invariably played with a straight bat. She never used untraceable poisons; she carefully checked the symptoms of overdoses, and was as accurate as to the availability and detection of these compounds as she could be. But there were a few notable exceptions. Serenite (A Caribbean Mystery), Benvo (Passenger to Frankfurt) and Calmo (The Mirror Crack’d from Side to Side*) are drugs that are pure Christie inventions, though the properties she attributed to them are very similar to those of barbiturate drugs. In fairness to Christie, she only used one of her invented drugs to kill a character, in The Mirror Crack’d from Side to Side; otherwise, these drugs were not critical to the plot.

Real-life inspiration

Agatha Christie did not just rely on an accurate and detailed knowledge of poisons. She read about real crime extensively, and was well versed in the sensational murders of the past. She referred to many real-life murders and poisoners in her books, killers such as Herbert Rowse Armstrong, Frederick Seddon and Adelaide Bartlett. She even used the circumstances of murder cases as inspiration for her plots.

The novel Ordeal by Innocence tells the story of Jacko Argyle, who was found guilty of murdering his mother. Years later, after Jacko had died in prison, a stranger turned up at the Argyle house with proof that Jacko was innocent. If Jacko did not kill his mother, who in the family did? The story was inspired by the Bravo case, a real-life poisoning that occurred in 1875. Charles Bravo married a wealthy young widow, Florence Ricardo, after a whirlwind courtship. Just four months into their married life Charles was taken ill after eating dinner with his wife and her live-in companion, Jane Cox. He died three days later after being attended by Dr James Gully, his wife’s former lover. Post-mortem analysis revealed he had been poisoned by a single dose of antimony. An inquest into the death decided on an open verdict, though it was widely suspected that Charles Bravo had committed suicide.

Subsequent reports in the press revealed that Jane Cox and Charles Bravo had been on bad terms, and that Cox had overheard an argument between the married couple over Florence’s association with Dr Gully. A second inquest was opened which effectively became a trial of the two women. A verdict of ‘wilful murder’ was returned, but with insufficient evidence to suggest who had administered the fatal dose of antimony. By this time the two women were no longer friends. The common suspicion was that Mrs Bravo had laced her husband’s wine with poison and attempted to cast suspicion on her companion. Charles Bravo’s murderer was never identified. To quote Agatha Christie, ‘And so Florence Bravo, abandoned by her family, died alone of drink, and Mrs Cox, ostracised, and with three little boys, lived to be an old woman with most of the people she knew believing her to be a murderess, and Dr. Gully was ruined professionally and socially.’ As Christie eloquently put it, ‘Someone was guilty—and got away with it. But the others were innocent—and didn’t get away with anything.’

Excerpted from A is for Arsenic: The Poisons of Agatha Christie by Kathryn Harkup. Copyright © 2015 by Bloomsbury USA. Reprinted by permission.