Anti-Retcon was a pill which could reverse memory loss. A possible side-effect was that it could cause the ingester "to remember Everything"; when Jack Harkness gave one to Ianto Janes, he cautioned that it wasn't selective and that he "might suddenly have a head full of maths tests and Monday mornings". It came from an alien source and took while to start working: usually three days, but sooner if there was a trigger. Ianto thought it tasted pleasantly fruity.
Jack gave one to Ianto to reverse his short-term amnesia after he awoke in the body of a woman. While he hoped he had given the correct pill, Jack asked "let me know if you start seeing clowns". (PROSE: Almost Perfect)