can someone with a drug addiction can ever change?

My mother has had a severe addition to cocaine for many years. My sister and I, as well as my family has always been there and tried to be supportive. She says that she is changing and will never touch it again. But we have heard that before. She recently got a new appartment and now all the sudden has new friends and we can hardly get ahold of her. She says that she is not doing cocaine again and that we need to trust her. Do I believe her or should I be a bit skeptical?