The chair is knocked over, and Stanley stumbles off. He realizes that he needs those meds (that may or may not exist) for his schizophrenia (that may or may not exist) before this reality (that may or may not exist) degenerates into chaos and madness.
He thinks to the voice that it is in fact not an addiction because schizo meds are not addictive and that he is clearly insane and clearly needs them to help his condition, and then chugs the milk... the walls melt away around him...