Retcon

Retcon, also known by the name of the pill that contained it, the amnesia pill, and occasionally by its more precise name, Compound B67, induced amnesia in anyone that took it. (TV: They Keep Killing Suzie) It was used by Torchwood Three to keep its operations secret from civilians. (TV: Everything Changes et al)

Effects
A side-effect of the pill was induced unconsciousness, though it took a while for this to occur. (TV: Everything Changes) Retcon appeared capable of erasing selective memories, or implanting false memories. When given to the party following alien disruption of the wedding of Gwen Cooper and Rhys Williams, it did not result in a large group of people forgetting that Gwen and Rhys were married. However, Jack said it was LV 6 Retcon, implying different levels of retcon invoked a different effect. (TV: Something Borrowed) In particular, Level 6 Retcon had a peculiar effect if mixed with champagne, causing a brief boost in activity before inducing sleep.

Retcon could erase memories, but didn't remove any other physiological effects; for example, retcon could remove the memory of taking a drug, but had no effect on the addiction, causing an addiction without an obvious reason. (PROSE: Kaleidoscope) It was possible to regain memories, and people were still capable of experiencing deja vu as when Gwen Cooper knew she had seen Suzie Costello somewhere before but only remembered the details after Suzie committed suicide. (TV: Everything Changes)

Overdose and abuse
Normally, a Retcon pill was meant for a single use to take care of an isolated incident that could compromise Torchwood's secrecy. Retcon was not designed to be used to regularly wipe the memory of an individual, or tamper with the psyche of a mindwiped individual by imparting a multitude of subliminal commands. Doing so could inflict severe psychological damage to a person's mind and drive them insane, or "reprogram" them into subconsciously doing the drugger's bidding. Such manic behaviour was forcibly invoked by Suzie Costello, when her attitudes towards Torchwood began to sour dramatically. She confided her experiences with the institute in Max Tresilian, a member of Pilgrim, a discussion group she had joined. Suzie was trying to rescue her sanity after bottling up the unpleasant nature of her job. However, she would resort to giving Max weekly doses of Retcon pills following each exposure of confidential information, which lasted over the course of two years. Inevitably, Suzie's hatred for her job peaked and led her to turn Max into a Trojan Horse that would let Suzie wreak vengeance on Torchwood posthumously if she had died, which would come to pass when she took her own life after she was exposed as a murderer. (TV: Everything Changes) Furthermore, it set in motion a convoluted scheme to resurrect herself and fully liberate her from the institute if she did indeed die. (TV: They Keep Killing Suzie)

There was a limit to the amount of retcon that one could take in a certain time before it became unsafe. (AUDIO: Another Man's Shoes) Under Yvonne Hartman, Xander Vaughn was given a large amount of the drug which resulted in him suffering serious brain damage and being confined to Whitecliff. (AUDIO: Cardiff Unknown - October 2018) Yvonne gave as much as possible to Andrea Davenport before it became dangerous in an attempt to save Andy Davidson's job. (AUDIO: Another Man's Shoes)

History
Sandra Pierce was retconned after her husband Tommy told her about his career at Torchwood One. (AUDIO: Retirement Plan)

Jack Harkness gave retcon to Gwen Cooper when they first met, but she later regained her memories. (TV: Everything Changes) Gwen gave retcon to Rhys Williams after confessing her affair with Owen Harper. (TV: Combat) Suzie Costello gave retcon to Max Tresilian once a week for two years. (TV: They Keep Killing Suzie)

Torchwood gave retcon to Danny Dillard to remove his memories of the Rehabilitator. He retained his addiction to it and turned to drugs to try to relive it. (PROSE: Kaleidoscope) Jack Harkness told Gwen to give Rhys retcon after he learned about Torchwood, but she refused. (TV: Meat)

The members of Torchwood 3 took retcon to forget Adam Smith, killing him. (TV: Adam) Torchwood 3 gave retcon to the guests at Gwen and Rhys' wedding. Rhys and Gwen were also offered the pills, but chose not to take them. (TV: Something Borrowed)

Idris Hopper took two retcon tablets to forget about Jack, Torchwood and the Ninth Doctor. (PROSE: The Twilight Streets)

In 2007, Jack was given retcon gas by his kidnapper in order to be able to take him to use his blood for the Miracle. Later the retcon gas was used on him and Gwen in order for them to forget that the gas was ever taken. (WC: Web of Lies)

Jack Harkness gave the drug to Esther Drummond so that she would forget what she had learned about Torchwood when they met. (TV: The New World) Jack forced a man who was watching Gwen's house to take retcon, so that he would forget that he saw Jack. (TV: The Gathering)

Ashildr used retcon during the early 21st century. She used it to wipe Rigsy's mind to hide the location of the trap street. (TV: Face the Raven)

Yvonne Hartman used retcon on Xander Vaughn. (AUDIO: Poker Face) She later used it to keep herself from being indicted in the investigation into the poor reaction to the Cardiff tsunami, a tactic that Jack thwarted with Bethan's assistance. (AUDIO: A Mother's Son)

Similar materials
Irving Braxiatel had access to a similar drug which caused a person (particularly a human) to lose the last twenty-four hours of their memory. (PROSE: The Empire of Glass)

The U.S. military possessed retcon gas, but it often caused subjects to go insane. (TV: Dreamland)

The Families used a gaseous form of retcon on Gwen Cooper and Jack Harkness in Chernobyl. (TV: Web of Lies)

River Song used Mnemosine recall-wipe vapour to erase some memories of her interactions with incarnations of the Doctor previous to the Tenth. (GAME: The Eternity Clock)

Behind the scenes
"Retcon" stands for Retroactive Continuity and has been used for some time, mainly in relation to role-playing games and comics. It refers to changing events from the past in order to make them fit with the current storyline.