the dispensation he has to refuse himself — "He kept trying to ask his…"
He kept trying to ask his heart if just codeine would be a relapse, according to the heart, but his heart was declining to comment.
Context
The test at its cruelest: the permission comes from the program itself, from Pat, who tells him the narcotics wouldn't be a relapse unless his own heart knew otherwise — handing the whole judgment back to the man least able, in toxemic agony, to trust his own motives. So Gately interrogates his heart about codeine and gets no answer, left to white-knuckle a refusal that nobody but him is enforcing, against a pain everyone around him is urging him to medicate.