Arsenic, cyanide, even nicotine: No toxic substance escaped the attention of Agatha Christie, the celebrated mystery writer of over five dozen novels. While her fictional victims were always subject to being stabbed, shot, or pushed off a cliff, the primary method of disposal was poison. Slipped into nightcaps, eye drops, even seeping from wallpaper, a variety of fatal chemicals provided her characters with mysterious ailments and puzzling clues that made for ideal murder material.

Christie’s assured handling of poisons came from first-hand experience with pharmaceuticals. She had volunteered to become an apothecary’s assistant at a hospital during both World Wars, acquiring a vast knowledge of drugs she then utilized in her detective fiction. With few exceptions, her descriptions of dosages, reactions, and mortality rates were rivaled only in specialist texts.

While this attention to detail was normally celebrated, there was one instance when a hysterical news media—and even Christie herself—became alarmed that she may have taken things too far.

The book in question was The Pale Horse, a novel about a group of contract killers using thallium, a heavy metal discovered in 1861 but largely obscure until Christie wrote about it. The real-life murderer was Graham Young, who was sentenced to life in prison for using thallium to poison an untold number of people, killing three. His experiments began in 1961, while he was just 14 years old. The Pale Horse, the first and only time Christie used thallium as a plot device, was published the same year.

At Young’s trial, pathologist Hugh Molesworth-Johnson said Christie’s descriptions of the drug were so accurate they rivaled the reference books of his profession, and the news media began to speculate about whether the boy had been influenced by the work. Had Christie's fiction turned into Young’s horrific fact?

Graham Young was said to be a peculiar little boy. Living in the London suburb of Neasden with his father, stepmother, and sister, Young largely kept to himself. He spent long hours in the library poring over medical texts and writing poetry. When he was 11, his father gave him a chemistry set as a reward for his fine grades.

His closest friend at school was a child named Christopher Williams. One day, Young offered Williams some cake. Williams became sick to his stomach but gradually recovered. Young kept notes on his symptoms.

As psychologists (and police) would later find out, his “friend” Williams had been Young's first human subject. Previously, he’d poisoned mice, insects, and plants with a variety of toxic substances easily acquired with his allowance and over the counter: antimony, belladonna, and thallium. The latter was a particularly malevolent chemical: odorless and tasteless, it’s treated by the body as potassium and creates significant damage in nerve cells. Numbness of the hands, slurred speech, and lethargy are common symptoms. In ingestion cases, small doses can build to lethal levels within two to three weeks. Victims who succumbed to it were often thought to have suffered from encephalitis or epilepsy.

When Young decided his stepmother, Molly, should get a dose, she may have become the first person in Britain to be intentionally poisoned with thallium. Young also slipped antimony—an emetic that causes copious vomiting—and other poisons to his father and sister. All three fell extremely ill. The family physician was at a loss until Young—by this point, a somewhat arrogant poisoner—brought some to school to show off. When the doctor learned from a teacher that Young was in possession of it, he had the boy's relatives tested.

All recovered save for his stepmother, who died. Young told psychiatrists the poisons had given him a sense of power.

“It grew on me like a drug habit,” he said, “except it was not me taking the drug.”

At the age of 14, Young was sentenced to Broadmoor, an asylum for the criminally insane, and was not expected to be released until near his 30th birthday. Doctors who evaluated the budding killer insisted he wouldn't hesitate to poison again at the earliest possible opportunity.

Eight years later, a different team of psychiatrists pronounced Young cured, despite the fact he was fond of growing nightshade—another poison—on prison property and once poured toilet detergent into the coffee of the nursing staff. Having entered incarceration a boy, he was released in 1971 as a fully-grown man of 23. Probation officers helped him land an interview for a job as a warehouse worker 30 miles north of London at the John Hadland Photographic Instrumentation company.

His would-be boss, Godfrey Foster, asked about the fact that this was his first job. Young replied he had suffered a nervous breakdown following the death of his stepmother. Foster phoned Young's psychiatrist to confirm the story; he was assured Young was fine. No mention was made of the fact that Young had poisoned his entire family and been imprisoned at Broadmoor, and no one seemed concerned that thallium happened to be a component in photographic lens manufacturing.

Young started work at Hadland in May 1971. Though they did manufacture lenses, no thallium (which can affect refraction in glass) was kept on the premises—a bit of irony perhaps only Young could have enjoyed at the time. However, he had no problem obtaining it at local drugstores.

Within a month of Young’s arrival, employees at Hadland began to fall ill. His supervisor, Bob Egle, got sick just before he was scheduled to take a vacation. He went ahead with his plans and began to feel better almost immediately, though he made no connection between his convalescence and the fact that Young was no longer serving him tea from a cart at work.

When he returned, his fingers grew numb and he began to stagger. After eight days in the hospital, he died of what doctors believed to be bronchopneumonia.

By this point, several other Hadland employees were feeling unwell. So many people began calling in sick that senior employee Fred Biggs agreed to come in on a Saturday. He sipped tea made by Young, and died 12 days later.

The epidemic led workers to believe Hadland was a toxic environment, possibly irradiated. Executives had a physician and a medical officer for the area come in and declare it safe. During one group meeting, the physician tried to calm everyone’s nerves. Young stood up and began peppering him with questions about the potential for heavy metal poisoning, particularly thallium.

The doctor thought Young was strange. His behavior compelled Hadland's investigators to reach out to the authorities, who examined his background and discovered Young had poisoned his household. Police searched his rented room and found a diary that offered explicit details of who he had poisoned, by how much, and their symptoms. Though Egle had been cremated by this time, forensic specialists were able to test his remains for thallium. The ashes were positive.

Young was sentenced to life in prison in 1972. During his well-publicized trial, much was made of Christie’s use of thallium in The Pale Horse and its relative rarity as a murder weapon. A month after his sentencing, Christie, then 81, expressed concern she could have given Young ideas. Her husband, Sir Max Mallowan, told reporters he wondered if “this fellow read her book and learned anything from it.” The Daily Mail published a list of similarities between Young’s victims and Christie’s descriptions. They could hardly resist the implication that the author had created a literal monster.

During Young’s 1972 trial, a pathologist testified that Christie’s book was the only source outside of reference books where such specific and accurate information about thallium could be found. Young himself never made any conclusive statement about The Pale Horse; it’s possible his knowledge came from studying medical texts during his library days as an adolescent.

Young, however, couldn't seem to escape his curious connection with Christie. When a nurse was reading the book in 1977 and recognized symptoms of thallium poisoning in a patient being treated in her ward, Scotland Yard suggested that doctors interview Young because he was undeniably an expert on the substance—and happened to be serving his life sentence right next door to the hospital, in Wormwood Scrubs Jail. It wasn't necessary, though; tests confirmed thallium. The patient was treated using a compound known as Prussian Blue, which binds to the metal and excretes it. She survived. Young died in 1990 of a heart attack at the age of 42.

It was not quite the end of thallium as a source of misery. In 2005, a 16-year-old in Shizuoka, Japan, used it to try to poison her mother, who fell gravely ill. While in the hospital, the girl—whose name was withheld from media—attempted to poison her again. She eventually confessed, with a judge sending her to reform school.

During their investigation, authorities discovered a blog that documented her mother’s systemic reactions. Among the other works cited in her journal: a biography about Young, and The Pale Horse by Agatha Christie.

Additional Sources: A is for Arsenic: The Poisons of Agatha Christie