Volxemortserum, also called Voldemortserum, Volxemortserom, Volximortserum, Volremortserum and Volxemortseruem, is a potion mentioned several times in My Immortal.
In Chapter 32, we learn that Professor Sinister is addicted to Volxemortserum, though she does not seem to have any symptoms similar to those experienced by Snap. In Chapter 34, Professor Sinister drinks the serum while sending Ebony back in time.
In Chapter 36, Ebony and her friends see Professor Sinister drinking the serum again. Draco suggests that they buy some "portions" in town to cure her addiction.
However, in Chapter 38, Ebony does not appear to know how to cure Voldemortserum addiction, as she asks Tom Satan Bombodil (Voldemort) about the cure during her time in the eighties. Satan tells her she must use vampire blood to cure the addiction.
Presumably, this potion is vaguely based on Veritaserum from the original books, which is a potion that forces the drinker to tell the truth. This is evident when Snap is forced to tell all his "secretes", which is probably meant to be "Secrets".