Retcon

Compound B67, also known as Retcon and occasionally the Amnesia pill was a pill that would induce amnesia in anyone that took it. (TW: They Keep Killing Suzie) It was used by Torchwood 3 to keep secret its operations from civilians. A side-effect of the pill was induced unconsciousness though it took a while for this to take effect. (TW: Everything Changes) Retcon appeared capable of only erasing selective memories, or allowing the implantation of false memories. When given to the wedding 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. (TW: Something Borrowed)

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. (TWN: Kaleidoscope) It was possible to regain memories and people were still capable of experiencing Deja Vu as shown when Gwen Cooper knew she had seen Suzie Costello somewhere before but only remembered when and where she'd seen her after Suzie committed suicide. (TW: Everything Changes)

Similar materials

 * Irving Braxiatel had access to a similar drug which caused a person (particularly a human) to lose the last 24 hours of their memory. (MA: The Empire of Glass)
 * The U.S. military possessed retcon gas, but it often caused victims to go insane. (DW: Dreamland)

Behind the scenes

 * "Retcon" stands for Retroactive Continuity and has been used for some time, mainly in relation to roleplaying games and comics. It refers to the fact of changing events from the past in order to make them fit with the present storyline.