Given enough time, they will clear their body of the substance (eliminiating physical reliance). Through honest introspection, they can realize that the drug doesn't do anything to help in the short term, is damaging in the long term, and that they don't really *WANT* to use because it's not actually fulfilling the need they have. Still, they know that their addiction is still there, and always will be. The problem is that an addiction at its core is a really strong habit. We can't actually "break" or forget habits. They'll ALWAYS be there as the brain (on a subconscious level) will NEVER forget what a fast, easy, and powerful hit of dopamine the drug was. The best addicts can do is try to replace the bad habit with a better one that gives them what they're looking for. You get rid of the "persons, places, things" that trigger the habit, and you establish new "persons, places, things" to build the replacement habit (examples: excercise, starting a business, running a volunteer program). Even if the new habit sticks, if you start bringing in the old habit elements from the addiction and start taking away the new habit elements, the addict will white-knuckle until they finally relapse (or transfer and develop another negative habit if it's an easy and powerful source of dopamine).
[If anyone reading this is interested about the subject of habit formation - but has no interest in bullshit 12-step/addiction literature - Charles Duhigg wrote a self-improvement book called "The Power of Habit" that's really interesting. If you don't want to buy it, you can find pdf and epub versions here:
https://oceanofpdf.com/authors/char...usiness-download-58827748320/?id=000669451329 ].
And it's even worse, because addicts don't just 'start over' with their addictions. A saying I've heard people use is "When I'm in recovery, my addiction is in the background doing pushups". If the level of destruction of their habit was a 7/10 after 5 years of using, and then they got sober for 10 years and then relapse, it won't take them 5 years to get back to where they were. They'll likely be back at 7/10 within a span of months, and then continue to worsen. Addictions are really nasty shit.