Retcon

Compound B67, also known as Retcon and occasionally the Amnesia pill would induce amnesia in anyone that took it. (TW: They Keep Killing Suzie) It was used by Torchwood 3 to keep its operations secret from civilians. A side-effect of the pill was induced unconsciousness though it took a while for this to occur. (TW: 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. (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 when Gwen Cooper knew she had seen Suzie Costello somewhere before but only remembered the details 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 twenty-four hours of their memory. (MA: The Empire of Glass)
 * The U.S. military possessed retcon gas, but it often caused subjects 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 changing events from the past in order to make them fit with the current storyline.