Looking for a book about a drug that causes habitual users to cross to another world that has an extra color

I've been trying to find a book that I read in the early 90's, but that judging from the cover was published in the late 60's/early 70's about a drug epidemic that was actually backed by the UN because of overpopulation. If you take enough of the drug you disappear, but end up in another world that has an extra color.