What goes on in the brain of a drug addict?

I have a family member who is in a lot of trouble. My family member is abusing Opioids (was/is taking it for severe, chronic pain, but, now they’re abusing it).

I want to try to understand what goes on in an addicts mind? What do they tell themselves? Do they lie to themselves?

Feel free to PM me, if you don’t feel like discussing it here.

Thanks..