Definition of "yage"

Maybe I will find in yage what I was looking for in junk and weed and coke. Yage may be the final fix.

1953, William Lee [pseudonym; William S. Burroughs], Junkie, New York: Ace Books

A large dose of Yage is sheer horror. I was completely delirious for four hours and vomiting at 10 minute intervals.

1953 April 12, William S. Burroughs, “To Allen Ginsberg”, in Oliver Harris, editor, The Letters of William S. Burroughs, 1945–1959, New York: Penguin, published 1993, page 155