obscenely pleasant, against his will
The air in the room got overclear, a glycerine shine, colors brightening terribly. If colors themselves could catch fire.
Context
The terrible datum the whole recovery arc has to reckon with: the forced dose, delivered in a room of horror by a man who's beaten and threatened him, still works — colors catching fire, the body lifting into a disembodied ease that is "obscenely pleasant." The mind can refuse and mow the truth and white-knuckle for months, and the Substance will still deliver on its promise the instant it crosses the barrier, because the disease lives in the flesh below the will. The obscenity is precisely that it's pleasant, that the drug keeps its side of the bargain even now.