Nora Volkow, in the first two sentences of this post, provides more clarity than I think I’ve every seen on the matter of choice and the brain disease of addiction:
Choices do not happen without a brain—it is the mechanism of choice. The quality of a person’s choices depends on the health of that mechanism. However much we may wish that a person’s choices were free in all instances, it is simply a fact that an addicted person’s failures in the realm of choice are the product of a brain that has become greatly compromised—it is readily apparent when we scan their brains. Even if taking a drug for the first time is a “free” choice, the progression of brain changes that occurs after that involves the weakening of circuits in the prefrontal cortex and elsewhere that are necessary for exerting self-control and resisting the temptations of drug use. Once addiction takes hold, there is greatly diminished capacity, on one’s own, to stop using. This is why psychiatry recognizes addiction as a disease of the brain, and why professional intervention is needed to treat it in most instances.