Agatha Christie’s Poisons
In the world of Agatha Christie, murder rarely makes itself known. Many a time, the death of a victim is certified as natural or accidental and friends and family are assured that there has been no foul play—until a sharper mind notices what does not fit. Poisons made this frequent pattern possible. Unlike guns or knives, they leave no obvious wound, often imitate natural disease, and require careful interpretation of symptoms rather than direct physical evidence. Christie, who trained as a hospital dispenser—or what would now be called a pharmacy assistant—during World War I, exploited these qualities with unusual precision. Over her long career she returned again and again to poison as her preferred instrument of murder, drawing on substances that were familiar, medically plausible, and deceptively easy to overlook. The result is the murder of more than 30 characters by poison in about 70 detective novels.
This list presents seven of the more well-known among the roughly 30 chemicals that Agatha Christie used to bump off her characters, as well as three she invented. Her command of toxicology is evident in the way she presented her characters’ symptoms and the modes in which the poison was delivered. The success of her stories can also be attributed in part to the weak regulation of these chemicals and limited investigative methods at the time, which made such crimes seem believable.
(The list entries contain spoilers.)
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Inorganic poisons
Arsenic
Occurring naturally in Earth’s crust, this widely distributed element is most toxic when it forms compounds with other elements. Exposure to inorganic arsenic compounds is particularly dangerous, and ingesting arsenic trioxide (As₂O₃), also called white arsenic, is especially deadly. Despite this, white arsenic was widely available in 19th-century England, where it was sold cheaply as a pesticide, and its use in tonics continued until the 1950s. Unsurprisingly, it became a favored murder weapon of enterprising killers. This white, odorless, and tasteless powder was easy to conceal in food and drinks and caused rapid, acute poisoning. In the body arsenic disrupts essential cellular processes, including respiration, energy production, and DNA repair, and damages the gastrointestinal tract, heart, brain, and kidneys. Symptoms of arsenic ingestion include nausea, vomiting, diarrhea, and stomach ache, which are often mistaken for gastritis, making poisoning easy to disguise.
This confusion is central to the principal murder in Christie’s 1938 novel Murder Is Easy. Her understanding of toxicology also appears in 4:50 from Paddington (1957), where a character notes that the symptoms of arsenic poisoning and those of gastroenteritis “are very much alike.” In both novels arsenic is added to strongly flavored foods or drinks—tea in Murder Is Easy and curry in 4:50 from Paddington—making detection by the victim unlikely. Successful or attempted murder through arsenic poisoning appears in several other works, including the short stories “The Cornish Mystery” (1923) and “The Lernean Hydra” (1939).
Cyanide
Inorganic compounds that contain a cyanide anion (an atom of carbon triple bonded to an atom of nitrogen) are called cyanides. They are fast-acting acute poisons and may occur as gases, liquids, or solids. Hydrogen cyanide (HCN), a volatile, colorless liquid widely used in industry, dissolves in water to form hydrocyanic (or prussic) acid. In both gaseous and liquid forms, HCN is highly toxic; it prevents the body from using oxygen, leading to breathing difficulty, vomiting, abnormal blood pressure and heart activity, loss of consciousness, and eventually death. Some plant parts, including apple seeds, peach and cherry pits, apricot kernels, cassava, and bitter almonds can release HCN in the human intestine when acted on by specific enzymes. However, the amount required to reach a lethal dose is extremely high, making accidental poisoning from consuming these plant products rare (see also Can Apple Seeds Kill You?). Salts of HCN, such as potassium cyanide (KCN) and sodium cyanide (NaCN), release HCN when they come into contact with water, acids, or carbon dioxide and produce the same toxic effects. These white, powdery substances can be mistaken for table salt or sugar, which makes them easy to administer in drinks or even package with powdered medicines, as was done with the pain reliever Tylenol that led to multiple deaths in the Chicago area in 1982.
In Sparkling Cyanide (1945) this very method is suspected when evidence shows that the victim had accepted the offer of a cachet faivre—a powdered painkiller usually sealed in rice paper or gelatin to hide its taste—before her death. Cyanide is later slipped into champagne to kill another victim. Although champagne could not have concealed the poison’s bitter taste or its almondlike smell, many people are unable to detect its odor, which helps explain its effectiveness and suitability for the plot. Christie uses this method of murder in other works, including And Then There Were None (1939) and the short story “Accident” (1929).
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Organic poisons
Strychnine
An alkaloid derived from the nux vomica tree (Strychnos nux-vomica), strychnine has long been used to kill avian and mammalian pests, particularly rats. In the 19th and early 20th centuries it was also popular as a stimulant and aphrodisiac, and it became the first performance-enhancing drug used in the modern Olympics. Although its use in human medicines was phased out by the late 20th century, strychnine occasionally appears as an adulterant in street drugs such as amphetamines, LSD, cocaine, and heroin. Even small amounts of this colorless (or white), odorless, bitter poison can cause convulsions, muscle spasms, and breathing difficulties when inhaled, ingested, or injected. Severe poisoning is fatal, and death occurs within hours of exposure, usually due to respiratory failure caused by sustained spasms of breathing muscles. Strychnine acts by interfering with the functioning of glycine, a chemical that helps regulate nerve signals to muscles. The result is overstimulation of motor nerves in the spinal cord, leading to uncontrolled muscle contractions all over the body. These often produce a rigid, arched back, clenched jaws, and the so-called sardonic smile (risus sardonicus) caused by facial muscle spasms.
The murder victim in Christie’s debut novel The Mysterious Affair at Styles (1920) shows these symptoms, appearing “to rest upon her head and her heels, with her body arched in an extraordinary manner,” before dying of strychnine poisoning. The method of administration relies on the poison’s distinctive chemistry, reflecting Christie’s detailed knowledge of the subject: With the addition of potassium bromide, strychnine in the victim’s medicine precipitates out of the solution and settles at the bottom of the bottle, where it is swallowed by the victim with the last dose of the medicine. Christie also uses strychnine in several other works, including the short stories “The Coming of Mr. Quin” (1924) and “How Does Your Garden Grow?” (1935).
Chloral
Also called trichloroacetaldehyde, chloral is a simple organochlorine compound and an aldehyde. When dissolved in water, it forms chloral hydrate, the first synthetically produced sedative-hypnotic drug. Because it was relatively easy to manufacture, chloral hydrate was widely used in the 19th and 20th centuries to treat insomnia and as a chemical restraint in psychiatric hospitals. Regular use of the drug often led to dependence. Its sleep-inducing effects were also misused for criminal purposes; it was given to unsuspecting victims as “knockout drops” or “Mickey Finns.” Chloral hydrate acts by enhancing the effects of gamma-aminobutyric acid (GABA), a neurochemical that reduces neuronal activity. This results in calming, sedation, and hypnosis. In overdose, however, it can cause extreme drowsiness progressing to coma, low blood pressure and body temperature, respiratory failure, and death.
Although chloral hydrate is now largely obsolete as a prescription sleep medication, it was still in common use when Christie published The Seven Dials Mystery (1929). At the time, a fatal overdose could be easily mistaken for an accident or suicide. Christie’s characters exploit this ambiguity in several other works, including The Secret Adversary (1922) and Why Didn’t They Ask Evans? (1934).
Digitalis
Digitalis refers both to the foxglove genus and to a group of cardiac glycosides derived from the dried leaves of these plants. These compounds strengthen contractions of the heart muscle. The medicinal value of foxglove extracts may have been known to the ancient Greeks, but definitive written records do not appear before the early 16th century. Digitalis entered prescription medicine in the mid-18th century, when it was used to treat edema and epilepsy. Today, digitalis drugs are closely associated with heart treatment and are used for conditions such as heart failure and chronic atrial fibrillation. They cause sodium and calcium buildup in the heart, resulting in stronger heart contractions that lead to increased cardiac output and more efficient blood circulation. These drugs also lower the heart rate. The most commonly prescribed digitalis compounds today are digoxin (formerly digitalin) and digitoxin. An overdose of either can cause digitalis toxicity, marked by vomiting, breathing troubles, unconsciousness, irregular heartbeat, and heart failure.
In Christie’s Appointment with Death (1938), the victim is deliberately injected with digitalis, ensuring a rapid onset of harmful effects. In the absence of modern forensic techniques, such a murder could easily be passed off as an accidental overdose of a prescribed drug. A related confusion, this time between sage leaves and foxglove leaves, forms the basis of the murder in the 1930 short story “The Herb of Death.”
Morphine
Infamous for its addictiveness, morphine is a narcotic analgesic (pain-relieving medicine) derived from opium. It is also the precursor of an even more addictive drug, heroin. Morphine is usually administered as hydrochloride, sulfate, acetate, and tartrate salts, available as colorless crystals or a white crystalline powder, both with a bitter taste. Its use for treating moderate-to-severe pain is strictly regulated in many countries, though misuse of the drug—along with that of other prescription opioids—is common. Morphine works by blocking pain signals as they travel from the body to the brain. Patients and people who use the drug may also experience euphoria, relaxation, sedation, and slowed breathing. This suppression of respiration is what makes morphine overdoses fatal. Breathing may stop entirely during morphine-induced slumber, leading to death from respiratory failure.
Such painless death is ensured by the murderer in Taken at the Flood (1948), in which the irony is taken further when morphine is substituted for bromide, historically used as a calming drug, in the victim’s prescription sleeping powders. By adding the poison to a box containing the sleeping powders, Christie introduces uncertainty about whether the victim will die, and, if so, when. Morphine appears in several of her works, including Sad Cypress (1940), Hickory Dickory Dock (1955), and By the Pricking of My Thumbs (1968). Obtaining morphine without a prescription remains difficult today, much as it was when Christie wrote those novels. Unsurprisingly, in all of these works morphine is sourced from places or by persons with a direct connection to medicine and caregiving.
Veronal
Veronal was a trade name of 5,5-diethylbarbituric acid, a barbiturate. Chemically called barbital in the United States and barbitone in the United Kingdom, it was widely used as a hypnotic and sedative in the first half of the 20th century. Veronal acts by increasing the activity of the neurotransmitter gamma-aminobutyric acid (GABA), which reduces the ability of nerve cells to fire. This slows brain activity and causes drowsiness. Its effects can also include mild euphoria, reduced anxiety, and lowered inhibitions, resembling the effects of alcohol intoxication. During the Jazz Age in England, veronal was a popular sleeping drug and could be obtained easily, often without a proper prescription. By the 1970s it was being replaced by safer alternatives, but not before gaining notoriety for accidental and intentional overdoses across the Western world. Fatal overdose typically involved slowed breathing; fluid buildup in the muscles, lungs, and brain; rising carbon dioxide levels in the blood; and coma. Death most often resulted from respiratory arrest and heart failure. When taken with alcohol, the risk was especially high.
In Lord Edgware Dies (1933) a character experiences these effects firsthand, though she does not live to tell the tale. The poison is given with alcohol, which both increases its potency and masks its bitter taste. Because the victim does not use the drug habitually, she has no tolerance to veronal, so a small dose is enough to kill her. Veronal also appears as a means of self-destruction, ironically, by the murderer himself, in the 1926 novel The Murder of Roger Ackroyd.
Imagined poisons
Lauded by professionals for the accuracy with which she presented her poisons, Christie is also known to have made up some chemicals for the sake of her stories. Even with these imagined chemicals—the blood pressure pill serenite in A Caribbean Mystery (1964), the hypnotic sedative drug calmo in The Mirror Crack’d from Side to Side (1962), and the benevolence-inducing chemical benvo in Passenger to Frankfurt (1970)—she went beyond just giving names. Experts believe that the properties of these chemicals that Christie invented resemble those of barbiturates.