Finger-Licking Poison

Poisons in detective novels are one of the great romances in fiction. Writers of the genre are always looking for new ways to administer the deadly stuff, which gives us this trope: a murder method wherein poison is unwittingly self-administered when the victim licks an object coated in the poison.

The object could be anything, although the more innocuous, the better. One popular method is to coat the pages of a book in poison, so that when the victim licks their finger to turn the page, the poison is passed to them. This is somewhat dated, since people don't lick their literature much anymore, but it may be Justified by saying the pages were stuck together or from an old, delicate book.

Other seemingly innocent lickable objects, such as stamps or envelopes, are also popular.

Most often a mystery trope, considering the murder angle. Modern examples of are probably inspired by the novel/film The Name of the Rose, although one early example is from the Arabian Nights.

Related to, but separate from, Poisoned Weapons. The murderer may use a Perfect Poison to do the job. Not to be confused with a Fingertip Drug Analysis, though less-subtle examples may rely on the victim trying one.

Literature

 * The Name of the Rose: In an attempt to keep hidden, an evil monk poisons the page corners so anyone who reads it will die before they can tell others about it.
 * Older Than Print: The Arabian Nights tale The Tale of the Vizier and the Sage Duban, wherein the Duban, sentenced to execution by a treacherous king, gives him a book with orders not to read it until after his head has been cut off. After that's done, the head comes back to life and instructs the king to turn three pages with his left hand. When the king turns the pages (naturally, licking his finger along the way) and finds nothing written there, the Duban essentially tells him the pages were poisoned and if the Duban had to go, he was taking the king with him. Crowning Moment of Awesome for a severed head. (Unfortunately, this makes the vizier who caused the execution a Karma Houdini, as he didn't touch the book and sure as hell wouldn't after seeing what happened to his king.)
 * In the Discworld novel Feet of Clay, this is one of the theories as to how got poisoned, in an obvious shoutout to The Name of the Rose.
 * This technique is mentioned in Bridge of Birds, with the added element that the books in question were pornographic.
 * Classic example: in Alexandre Dumas' La Reine Margot (AKA Marguerite de Valois), poisoned book is used in attempt on King Henry of Navarre's life, but the plan backfires with disastrous results. Earlier in the same novel, even more devious plan to poison Henry via his paramour's lipstick is employed.

Live-Action TV

 * 11th Hour used this one early in its first (only?) season.
 * In the CSI New York episode"Page Turner", the killer coats the pages of a book in thallium to poison his victims.

Western Animation

 * In the old TV show, Jacob Two Two, a bad guy puts into motion a plot to kill people via this method.

Anime and Manga

 * In Case Closed, a man was murdered via.
 * One of the movies had a woman murdered in a similar way.
 * Another example: a musician is murdered by  and asked to perform a song that required him to throw off the jacket and take a pose where the poison would transfer to his hand. The food, all non-poisoned, were things like sushi and nigiri and there were no eating utensils.
 * Another man was poisoned via a poisoned wet napkin while eating sushi.
 * And YET ANOTHER was poisoned because his murderer put poison on the spot on the lazy susan between two dishes that the victim was allergic to, and so he kept getting poison on his fingers every time he saw said dishes.
 * A man who disliked sour things was nevertheless poisoned by a lemon wedge in his drink because, as part of a marketing gimmick, he had eaten a miraculin berry and was unable to taste the sourness.

Comic Books

 * One Batman comic featured the Joker's venom being applied to postage stamps.
 * In the classic "The Judas Contract" storyline in Teen Titans, Deathstroke captures Gar 'Changeling' Logan by drugging the gum in the envelopes Gar is using to respond to his fan mail.

Film

 * Not exactly lickable, but in the movie The Mechanic, Arthur Bishop.
 * Another not exactly lickable example occurs in A Game of Shadows when

Literature

 * In Discworld stamp-collecting, the original Assassins' Guild 3p stamp (the Thrupenny Dreadful), is very rare, at least gummed. The in-universe reason for this is that they were recalled due to unsubstantiated rumours that the Guild was using it to fulfill contracts.
 * In Robin Hobb's Assassin's Apprentice, one of Fitz-Chivalry's assassination ploys was to
 * During the Mallorean series, Sadi kills a man by coating the man's soup spoon with poison.
 * A Song of Ice and Fire: twice, with the same method. First the prologue of A Feast for Crows:  Second,.
 * The Three Widows, by Ellery Queen had a victim being slowly poisoned even though everything she ate and drank was carefully screened beforehand. It turned out the would-be killer was  and the poison was
 * Non-poison example: In Red Seas Under Red Skies, Locke and Jean win a card game by sprinkling a sleep-inducing drug on the cards. One of their opponents is notorious for eating and licking her fingers while she plays, and she forfeits the game when the drug puts her to sleep.

Live-Action TV

 * There was a Remington Steele episode where the poison was in the glue on some envelopes Steele and Laura were expected to lick.
 * In Seinfeld George's fiancee is accidentally poisoned by the cheap glue on the wedding invitation envelopes, because George was too stingy to pay for better ones (and too lazy to seal any invitations himself).
 * In the Jonathan Creek episode "The House Of Monkeys" the victim was sent a request for a signed copy of his book. The murderer included a stamped addressed envelope to send the book in... stamped, addressed and poisoned with a psychotropic drug on the flap you lick.
 * CSI did this one—during the course of an attempted murder, the apparent victim spilled ricin on her pen, and then killed herself by biting the end of it.
 * In the Criminal Minds episode "Poison," the team discovers that groups of people who had been slipped LSD had ingested it by licking bank envelopes whose seals were coated in the drug. Later in the same episode, the UnSub tries to poison a group of people with botulism via the same method.
 * In one episode of Benson, a person with a habit of sucking on the earpiece of his reading glasses was killed by poison placed on the earpiece.

Tabletop Games

 * In the Dungeons & Dragons adventure series Curse of the Crimson Throne,

Video Games

 * A poisoned letter stamp in Apollo Justice: Ace Attorney. Also upgraded to poisoned nail polish, because the intended victim,, had a tendency to bite her nails a lot.
 * A Game at Dinner, an in-game short story in some Elder Scrolls games, has Helseth implying to his assembled dinner guests that he put poison on the cutlery of someone he knows has been spying on him.
 * Finger-licking isn't explicitly mentioned, but in Assassin's Creed, Agostino Barbarigo dies after receiving several poisoned letters from the Assassins.

Real Life

 * Real-life example: While not highly toxic by itself, Dimethyl Sulfoxide (DMSO) has the property of being quickly and easily absorbed through skin contact, allowing easy contact absorption of whatever else happens to be mixed with it. This allows a deadly but hard-to-deliver poison to be easily ingested through the skin.