Mnemosine recall-wipe vapour

Mnemosine Recall-Wipe Vapour was a substance used to induce amnesia or erase memories in a subject, similar to Torchwood Three's retcon pill. River Song used this when she would travel to see incarnations of the Doctor which came before the first one she met (from her point of view) so as not to contaminate the timeline. (VG: The Eternity Clock)