Poisoned candy myths

Poisoned candy myths are urban legends about malevolent strangers hiding poisons or sharp objects such as razor blades, needles, or broken glass in candy and distributing the candy in order to harm random children, especially during Halloween trick-or-treating. These stories serve as modern cautionary tales to children and parents and repeat two themes that are common in urban legends: danger to children and contamination of food.
No cases of strangers killing or permanently injuring children this way have been proven. Commonly, the story appears in the media when a young child dies suddenly after Halloween. Medical investigations into the actual cause of death have always shown that these children did not die from eating candy given to them by strangers. However, in rare cases, adult family members have spread this story in an effort to cover up murder or accidental deaths. In other incidents, a child who has been told about poisoned candy places a dangerous object or substance in a pile of candy and pretends that it was the work of a stranger. This behavior is called the "copycat effect". Folklorists, scholars, and law enforcement experts say that the story that strangers put poison into candy and give that candy to trick-or-treating children has been "thoroughly debunked". 
Worries that candy from strangers might be poisoned has led to the rise of alternative events to trick-or-treating, such as events held at Christian churches, police and fire stations, community centers, and retail stores.
History.
Claims that candy was poisoned or adulterated gained general credence during the Industrial Revolution, when food production moved out of the home or local area, where it was made in familiar ways by known and trusted people, to strangers using unknown ingredients and unfamiliar machines and processes. Some doctors publicly claimed that they were treating children poisoned by candy every day. If a child became ill, and had eaten candy, the candy was widely assumed to be the cause. There were also actual cases where poison was added to sweets, for example the use of an arsenic based green pigment (Scheele's green) for decorations on cakes and blancmange. 
In the 1890s and 1900s, the US Bureau of Chemistry and other state agencies tested hundreds of kinds of candy and found no evidence of poisons or adulteration. These tests revealed that inexpensive glucose (from corn syrup) was in common use for cheap candies, that some candies contained trace amounts of copper from uncoated copper cooking pans, and that coal tar dyes were being used for coloring, but there was no evidence of the many types of poison, industrial waste, garbage, or other adulterants alleged to be present. Eventually, the claims that children were being sickened by candy were put down to indigestion due to overeating, or to other causes, including food poisoning due to improper cooking, hygiene, or storage of meat and other foods.
Social causes.
The prevalence and persistence of these myths during the 1960s and 1970s, a time of social upheaval, greater racial integration, and improved status for women, reflected societal questions about who was trustworthy. Because society was struggling with questions about whether to trust neighbors in newly integrated neighborhoods, or young women who were publicly rejecting the subservient, motherhood-focused roles previously assigned to women, these stories about unidentifiable neighbors allegedly harming random, innocent children during an event intended to bring happiness to these children caught and retained the public imagination in a way that accurate stories about a judgmental neighbor, an abusive parent, or an adult carelessly leaving harmful chemicals where children can reach them, wouldn't have. An academic view sees this as an example of a rumor panic, with halloween developing as a carnival-like folk institution – meant to release social tensions – losing its functionality as neighborhoods themselves break down (for various reasons).
Effects.
Due to their fears, parents and communities restricted trick-or-treating and developed alternative "safe" events, such as trunk-or-treat events held at Christian churches. This collective fear also served as the impetus for the "safe" trick-or-treating offered by many local malls.
This story also promoted the sale of individually wrapped, brand-name candies and discouraged people from giving homemade treats to children.
Candy-tampering myth.
Development of the modern candy-tampering myth.
Several events in the late 20th century fostered the modern-day candy tampering myth.
In 1959, a California dentist, William Shyne, gave candy-coated laxative pills to trick-or-treaters. He was charged with outrage of public decency and unlawful dispensing of drugs.
In 1964, a disgruntled Long Island, New York woman gave out packages of inedible objects to children who she believed were too old to be trick-or-treating. The packages contained items such as steel wool, dog biscuits, and ant buttons (which were clearly labeled with the word "poison"). Though nobody was injured, she was prosecuted and pleaded guilty to endangering children. The same year saw media reports of lye-filled bubble gum being handed out in Detroit and rat poison being given in Philadelphia, although these media reports were never substantiated to be actual events.
Another notable milestone in the spread of the candy tampering myths was an article published in "The New York Times" in 1970. This article claimed that "Those Halloween goodies that children collect this weekend on their rounds of 'trick or treating' may bring them more horror than happiness", and provided specific examples of potential tamperings.
Reports and copycat incidents peaked shortly after the Chicago Tylenol murders, which were first reported one month before Halloween in 1982. This incident involved a murderer who added poison to a few bottles of over-the-counter medication after the medication had been delivered to stores.
In 2019, the Massachusetts Cannabis Control Commission warned that "parents should be on the lookout for products that are unsafe for kids after they trick-or-treat."
Debunking the myths.
Joel Best, a sociologist at the University of Delaware, specializes in the scholarly study of candy-tampering legends. He collected newspaper reports from 1958 to 1983 in search of evidence of candy tampering. Fewer than 90 instances might have qualified as actual candy tampering. In none of the cases does he attribute the events to "random attempts to harm children" at the Halloween holiday. Instead, most cases were attempts by adults to gain financial compensation or, far more commonly, by children to get attention. Best found five child deaths that were initially thought by local authorities to be caused by homicidal strangers, but none of those were sustained by investigation.
Fabrications by children are particularly common. Children sometimes copy or act out the stories about tampered candy that they overhear, by adding pins to or pouring household cleaners on their own candy and then reporting the now-unsafe candy to their parents. In these incidents, the children have not been harmed; they know that the dangerous item is present and that it would be unsafe to eat the candy.
Far more prevalent during the same period were reports of vandalism, racist incidents, or children being injured in pedestrian–vehicle collisions on Halloween.
Misattributed poisoning deaths.
The deaths of five children were initially blamed on stranger poisoning. However, none of these claims were sustained upon investigation.
In a 1974 case, an 8-year-old boy from Deer Park, Texas, died after eating a cyanide-laced package of Pixy Stix. A subsequent police investigation eventually determined that the poisoned candy had been planted in his trick-or-treat pile by the boy's father, Ronald Clark O'Bryan, who also gave out poisoned candy to other children in an attempt to cover up the murder, though no other children consumed the poisoned treats. The murderer, who had wanted to claim life insurance money, was executed in 1984. In this case, the distribution of poisoned candy is true, but no children other than the intended victim were harmed.
Media and the myth.
Despite these claims of poisoned candy being eventually proved false, the news media promoted the story continuously throughout the 1980s, with local news stations featuring frequent coverage. During this time cases of poisoning were repeatedly reported based on unsubstantiated claims or before a full investigation could be completed and often never followed up on. This one-sided coverage contributed to the overall panic and caused rival media outlets to issue reports of candy tampering as well. However, Joel Best says that the spread of the myth cannot be blamed solely on the media, and that it must have been transmitted via word of mouth as well.
By 1985, the media had driven the hysteria about candy poisonings to such a point that an ABC News/"Washington Post" poll that found 60% of parents feared that their children would be injured or killed because of Halloween candy sabotage.
Advice columnists entered the fray during the 1980s and 1990s with both "Ask Ann Landers" and "Dear Abby" warning parents of the horrors of candy tampering:
"In recent years, there have been reports of people with twisted minds putting razor blades and poison in taffy apples and Halloween candy. It is no longer safe to let your child eat treats that come from strangers." –Ann Landers in 1995
"Somebody's child will become violently ill or die after eating poisoned candy or an apple containing a razor blade." –Dear Abby in 1983
Candy tampering by friends and family.
Almost all tampering cases – at a rate of one or two per year – involve a friend or family member, usually as a prank. Almost all of those involved sharp objects, rather than poisoning. Three-quarters of them resulted in no injuries, and the rest resulted in only minor injuries. No child has ever been killed by eating a Halloween candy from a stranger.
Some marijuana edible candy is intentionally manufactured to appear similar to regular candy but is easily distinguished from commercially available regular candy. Concerns have been raised that children may easily confuse the two. St. Louis DEA claimed to have found marijuana edibles distributed on Halloween in 2017 but has not presented a case as of October 2018.