"Religion [. . .] is the ____ of the people” (Karl Marx) (5)

I believe the answer is:

opium

(Other definitions for opium that I've seen before include "where horse originally came from?" , "Duller" , "Poppy-based drug" , "wars with China" , "Addictive drug made from poppies" .)

I've seen this clue in The Guardian and The Times.

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