Yen pox is the name given to opium ashes. After smoking the opium, one will be left with a dark substance that can be eaten or dissolved in water and then injected intravenously. It is said to contain the same quantity of morphine as before smoking the little opium ball.
The first time I saw this term used was in the novel Junky, by William S. Burroughs, but an older friend of mine later told me that pushers used to sell yen pox on the street in the 40s-50s.
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