Mnemosine recall-wipe vapour was a substance used to induce amnesia or erase memories in a subject, like Torchwood Three's retcon pill or a memory worm; given its name, it was gaseous. River Song used it when she went to see incarnations of the Doctor which came before the first one she met (from her point of view) to not contaminate the timeline. (GAME: The Eternity Clock)
Unlike a similar drug, Retcon, it did not negatively affected the psyche of the person its used on if they are exposed to it repeatedly. (TV: They Keep Killing Suzie) However, since River tended to use this on the earlier versions of the Doctor, a Time Lord, its possible his superior biology and regeneration kept him from experiencing negative effects.