the medication that had stopped working
By this time it was hard to tell whether he’d been ingesting anything or not. Apparently at a certain point it stops making a difference.
Context
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.