the medication that had stopped working — "that even work and Wild Turkey…"

The late stage where the substance no longer registers as either present or absent — Hal's flat report that "at a certain point it stops making a difference" — paired with the drinking named as exhausted medicine, work and Wild Turkey both having quit working. The drug that once medicated the dread has become indistinguishable from its absence, which is a definition of the end of a habit's usefulness.