Talwin is the corporate name of the street drug Sunshine.
The text proper concludes after Don Gately remembers taking Sunshine. There is speculation as to whether Gately's memory is triggered by the hospital administering narcotic pain-killers against his will (as they are performing a major operation at this point), or whether Gately is finally free from his addiction and has found--as impossible as the novel makes it seem--a "way out."
The novel physically ends with the endnote "Talwin-NX--®Sanofi Winthrop U.S." (p. 1079n388), closing the novel, as Stephen Burn points out, on a corporate note. This ending also contradicts the theory that Gately had in fact found a way out. The physical way out of the cyclical novel is a reference to Talwin; the door out of the cage of the novel's annular recursivity is perhaps only another cage--the cage of narcotics addiction.