For both Sara and Harry, their tendencies for addiction were seen in the opening shot of the film. He needs to steal the television to support his habit. She retrieves it back and locks it to the radiator to support hers. They both are shown to possess the characterization that would succumb to addiction.
Yet, I think that where the change in behavior to being susceptible to addiction and being controlled by it can be seen when things begin to go bad for both of them. When Harry's and Sara's respective predicaments take a turn for the worse, there is no recognition of the need to stop self- destructive behavior. Rather, they increase the force and magnitude of their addiction. When Sara recognizes that her perception is distorted in terms of things moving too fast or too slow, or when objects in her world begin to "bark" at her like the fridge and the television, she descends into taking pills with greater frequency. This becomes a distinct moment when it becomes clear that she has changed. She is no longer Mrs. Goldfarb, or Harry's mother, or even the likable woman who simply wants "to have a gnosh." She has become a creature of addiction.
Harry operates in much the same manner in showing how the change in his behavior has been caused by addiction. The love he has for Marion, the hopes and dreams he has for his mother and the life he wishes to have with Marion become supplanted by his need to feed his addiction. He encourages Marion to prostitute herself to "score" and no longer can provide the love he once had for her. It is interesting to note how far Harry has changed as a result of his addiction:
They held each other and kissed. And pushed each others' darkness into the corner. Believing in each others' light, each others' dreams.
This description is how Harry felt about Marion prior to his addiction take hold of him. When Harry realizes that his supply is evaporating, one sees that he no longer feels such a tender notion of love for Marion. There is nothing shared in the addiction that he experiences. He becomes self- serving and driven by the need to "score." This becomes a distinct change that might have been brought about because of his addiction.