Agatha Christie learned a lot about poisons from her work as a dispenser in a hospital pharmacy during the First World War so naturally the plot of her first book involved murder by poison. The poison she employed in "The Mysterious Affair at Styles" was strychinine.
"The Mysterious Affair at Styles" was published in 1920. In her autobiography, Agatha Christie wrote "since I was surrounded by poisons, perhaps it was natural that death by poisoning should be the method I selected". Her first victim, Emily Inglethorpe, died a very painful death – "The convulsions were of a peculiar violence, Dr Wilkins...they were quite tetanic in character". Strychnine is a poisonous alkaloid obtained from the seeds of the nux vomica tree. The symptoms include agonising convulsions and death is usually due to asphyxiation resulting from continuous spasms of the respiratory muscles.
In "A Pocketful of Rye", Rex Fortescue died of taxine poisoning which was administered to him at breakfast in his marmalade. Taxine is a poisonous alkaloid found in the foliage and berries of the yew tree, which grows in England.
Rex Fortescue lived at Yewtree Lodge, a house with "large numbers of clipped yew hedges", the source of the poison. "… she didn’t mean to murder anybody, but she put the taxine in the marmalade. She didn’t think it was poison, of course" (Miss Marple).
Rex Fortescue wasn’t the only victim in "A Pocketful of Rye". His second wife, Adele, was also poisoned, not by taxine, but by another poison which can also be obtained from plants. The murderer slipped which cyanide into her tea.
"Mrs Fortescue was still sitting on the sofa, dead. Beside her was a tea cup a quarter full and in the dregs of it was potassium cyanide." Cyanide can be obtained from the seeds of certain stone fruits such as apricots and wild cherries and the seeds of fruits of some members of the apple family.
Ella Zielinsky, actress Marina Gregg's social secretary, in "The Mirror Crack’d from Side to Side", died by inhalation of prussic acid.
Prussic acid is also known as hydrocyanic acid (a derivative of cyanide). Ella Zielinsky suffered from allergies and used a nasal spray for relief. "She inserted the nozzle into one nostril and squeezed. The warning came a second too late … her brain recognised the unfamiliar odour of bitter almonds … but not in time to paralyse the squeezing fingers …"
In "The Pale Horse", a very unusual form of poisoning (thallium) was employed. The symptoms of this type of metallic poisoning include nervous and gastrointestinal disorders and rapid loss of hair.
According to Dew’s Agatha Christie Mystery website, readers of "The Pale Horse" have subsequently recognised the symptoms of thallium poisoning, resulting in the saving of at least two lives. In one case, a nurse recognised the symptoms in time to save the life of an infant. In another case, a man was prevented from trying to kill his wife. This website also cites the case of a detective who realised thallium was the cause of the mysterious deaths of six other men. He subsequently identified the murderer, who later confessed to the crimes.
Another unusual form of poisoning was used in "Death in the Air" (also known as "Death in the Clouds"). While on a short flight, Madame Giselle, a French moneylender, is murdered by a poisoned thorn. Madame Giselle (also known as Marie Angelique Morisot) was poisoned by a thorn dipped in the venom of the South African boomslang snake. The venom of the boomslang is more potent than that of the cobra and causes death by haemorrhage. "The woman’s head lolled over sideways. There was a minute puncture mark on the side of her throat".
It is possible that many readers cheered when the nasty, sadistic Mrs Boynton was poisoned in "Appointment with Death". She was poisoned with an overdose of the heart medication known as digitoxin.
Digitalis is a substance obtained from the dried leaves of the common foxglove (Digitalis purpurea) and used as a drug that strengthens contractions of the heart muscle for patients suffering congestive heart failure. Digitoxin and digitalin are among the most commonly prescribed forms of digitalis. It must be prescribed and monitored with great care because there is not much leeway between the effective dose and a lethal dose. "And Mrs Boynton already suffered from heart trouble?" "Yes, as a matter of fact, she was actually taking a medicine containing digitalin." … "D'you mean … that her death might have been attributed to an overdose of her own medicine?"
In "Three Act Tragedy" (also known as "Murder in Three Acts"), Reverend Stephen Babbington, Sir Bartholomew Strange and Margaret De Rushbridger are all dispatched by a poisonous alkaloid derived from the leaf of a plant. The poison employed was nicotine.
Nicotine is found throughout the tobacco plant, particularly in the leaves. Depending upon the amount consumed, nicotine can cause nausea and vomiting, headache, stomach pains, convulsions, paralysis and death. "My goodness … I’ve only just realized it. That rascal, with his poisoned cocktail! Anyone might have drunk it. It might have been me". "There is an even more terrible possibility that you have not considered", said Poirot … "it might have been ME".
Coco Courtenay’s death was caused by a fatal dose of the addictive cocaine in "The Affair at the Victory Ball".
Cocaine is a white, crystalline alkaloid that is obtained from the leaves of the coca plant (Erythroxylum coca). When taken in small amounts, cocaine produces feelings of well-being and euphoria, along with a decreased appetite and increased mental alertness. When taken in larger amounts, and with repeated use, it can produce anxiety, depression, irritability, insomnia, chronic fatigue, mental confusion, paranoia, and convulsions that can cause death. "Her maid … admitted that Miss Courtenay was a confirmed taker of the drug and a verdict of accidental death was returned". Of course it wasn’t - but the murderer was not as clever as Poirot!