An interesting perspective.
Junk turns the user into a plant. Plants do not feel pain since pain has no function in a stationary organism. Junk is a pain killer. A plant has no libido in the human or animal sense. Junk replaces the sex drive.
Seeding is the sex of the plant and the function of opium is to delay seeding. Perhaps the intense discomfort of withdrawal is the transition from plant back to animal, from a painless, sexless, timeless state back to sex and pain and time, from death back to life.