Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
At the end of VN's novel Ada (1969) Dr Lagosse makes Van and Ada the last merciful injection of morphine:
Was it time for the morphine? No, not yet. Time-and-pain had not been mentioned in the Texture. Pity, since an element of pure time enters into pain, into the thick, steady, solid duration of I-can’t-bear-it pain; nothing gray-gauzy about it, solid as a black bole, I can’t, oh, call Lagosse.