Before Agatha Christie became one of the world’s bestselling authors, she worked in a chemist’s shop in Torquay, England. There, a man she identified in her memoir only as Mr. P carried curare in his pocket because it made him “feel powerful.”

Christie recognized that Mr. P’s behavior could make him dangerous, despite his “cherubic appearance.” She also caught him adding 10 times the required drug dosage while demonstrating how to infuse medications into suppositories.

Convinced he would ignore a “little girl” who challenged his mistake, Christie spilled the medicine and “trod on them firmly.” The intervention prevented the dangerously prepared drug from reaching the shop’s shelves, according to her memoir.

That unsettling workplace helped ignite the scientific curiosity behind Christie’s detective fiction. Across her career, she developed a deep understanding of poisons, medications and chemical combinations that shaped her mysteries and extended her influence beyond literature.

After leaving Mr. P’s shop, Christie began training as a pharmacist at Torquay’s hospital in 1916. While preparing for her examinations, she read “The Pharmaceutical Pocket Book — for Practitioners & Students,” which inspired the murder weapon in her 1920 debut novel, “The Mysterious Affair at Styles.”

The novel uses strychnine, a neurotoxin once administered in tiny doses as a stimulant, combined with other drugs to kill its victim. Christie’s personal copy of the pocket reference, marked with stains and her name, is among more than 200 objects featured in “Agatha Christie: A World of Mystery.”

The British Library exhibition in London opens October 30, according to lead curator Lucy Rowland. The artifacts, shared by Christie’s family and trust, demonstrate the extensive research supporting her work and her command of medications and lethal chemical mixtures.

Poison serves as the murder weapon in 41 of Christie’s 66 detective novels, and her descriptions remain accurate today. Former chemist Dr. Kathryn Harkup called Christie’s writing “stealth science communication at its peak,” praising her ability to convey complicated scientific ideas without distracting readers from the mystery.

Christie began her hospital service at age 24, volunteering as a nurse at Torquay’s Red Cross Hospital in 1914 during World War I. She treated wounded soldiers and trained while working, progressing from rolling bandages to caring directly for patients.

Although Christie wrote that she initially felt “disgraced” after becoming faint during an operation, she learned to control her reaction. In 1916, she moved into pharmacy work because the hospital dispensary was understaffed and needed help mixing medications.

Her preparation for the Society of Apothecaries’ Assistant Examination in 1917 sharpened her attention to detail. On one occasion, she awakened in a panic and returned to the hospital pharmacy to inspect every pot of ointment because she feared she had caused cross contamination.

Christie understood that a small measuring or mixing error could transform a useful medication into a lethal substance. That insight appears in “The Mysterious Affair at Styles,” when pharmacy volunteer Cynthia Murdoch warns how “fatally easy it is to poison someone by mistake.”

Her pharmaceutical precision earned recognition from “The Pharmaceutical Journal,” whose review of her first novel celebrated her informed use of strychnine. The reviewer delivered a compact verdict: “Miss Agatha Christie knows her job.”

During World War II, Christie studied again before working in the pharmacy at University College London. Chief pharmacist Harold Davis suggested thallium as a fictional poison, and Christie later combined that recommendation with memories of Mr. P while creating the sinister pharmacist in her 1961 novel, “The Pale Horse.”

Thallium is tasteless, odorless and slow acting, and it mimics potassium in the body, making poisoning difficult to detect. Christie accurately used hair loss as a clue, and her depiction has been credited with helping identify two poisoning cases.

In 1975, Christie received a letter from a young South American woman who said “The Pale Horse” helped her recognize that an acquaintance was being poisoned. The woman wrote that the victim would not have survived without the knowledge she gained from Christie’s novel.

In 1977, London nurse Marsha Maitland connected a sick 19 month old child’s symptoms to thallium poisoning after reading the same book. Testing confirmed the child had accidentally ingested thallium from an insecticide used in her home, and she fully recovered after four months of treatment.

The attending physicians later published a paper acknowledging Christie’s descriptions and Maitland’s familiarity with the literature. Christie never returned to pharmacy work after World War II, but she continued following medical developments through reading and communication with experts.

Her knowledge was formidable enough that historian Dr. Mark Aldridge said, “She could have gotten away with” murder if she had wanted to. Instead, Christie used that expertise to construct mysteries that gave readers every fact needed to solve the crime, even if most could not see the answer coming.

Her great grandson James Prichard said Christie’s medical interest grew from her wartime hospital service and remained with her throughout her life. Doctors and pharmacists repeatedly appeared in her fiction as victims and killers, reflecting how permanently medicine had entered her imagination.