Mnemosine recall-wipe vapour

Mnemosine Recall-Wipe Vapour was a substance used to induce amnesia or erase memories in a subject, like Torchwood Three's retcon pill; given its name, it's 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. It appears to not affect a person should it be used repeatedly on them. (VG: The Eternity Clock)