An insinuation that by beleiving some point, you must have taken drugs to come to this conclusion. It is derived from an act done by Jim" Jones (May 13, 1931 – November 18, 1978). He was the founder and leader of the Peoples Temple, which is best known for the November 18, 1978 death of more than 900 Temple members in Jonestown, Guyana along with the deaths of nine other people at a nearby airstrip in Georgetown. He had poisioned the Kool Aide and to show their faith, they drank it.
Rather than saying "Don't believe him." one would say, "Don't drink his kool aide."