system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is heroin so addictive? What does it do to the brain? Can anyone explain the feeling?
gpt
The human brain has a natural set of hormones that are used to moderate pain and emotional responses called endorphins, the opium poppy based drugs imitate the actions of endorphins, that ease pain and induce and pleasurable sensations.  The most potent of the naturally occuring "opiates" is morphine.  Hold that thought.

There is a chemical and physical structure in our body called the "blood brain barrier." The brain is a seriously important organ and is isolated from much of the rest of the body's circulation for its own protection, this is good thing but can have it's drawbacks. Certain poisons don't get to the brain, good, but antibiotics may be blocked as well.  Now, back to the opiates.

Morphine crosses the blood brain barrier, if eaten, smoked, or intravenously injected it gets into the brain to work its tricks.  Heroin (diacetylmorphine) is a drug made from morphine that crosses the blood brain barrier *much more effectively*, it is quicker to get into the brain and do what morphine does.  It's heroin in the little baggie from the dealer, it's heroin in the syringe and in the vein but it is morphine in the brain.

If you were perfectly healthy and wealthy and just as lazy as hell you might be able to just hire someone to carry you around to every anywhere you wanted to go, do anything that needed to be done, lift anything that needed to be lifted.  This would have serious effects on your body, you'd lose muscle mass and become weaker and weaker over time to the point where you'd not just *want* your helper to carry, do, and lift everything, you would *need* that help.  If your helper left you'd either hire a new one or struggle to get up off your ass and *recover* from the mess you'd made of yourself.  That's addiction for you.

Heroin (morphine) takes over for your natural endorphin system that is there for the internal control of pain and does all the work to the point where your body *needs* it to avoid pain.  When heroin is *withdrawn* your body goes into *withdrawal* and **it hurts like hell.**

Feels real good when you start, up to the point of addiction, and is quite painful to quit after that.

Edit: minor spelling.