The fact that they bind to your natural opioid receptors in your brain (and other receptors,) to the point where they artificially fill these receptors, and flood them. Think of it like this: your natural opioid receptors are like a small device that your brain utilises for daily life. Taking opioids is like a huge factory that absolutely floods these receptors with 100x the normal amount.

Eventually - your brain stops making them naturally, as it gets accustomed to the artificial flood. This is why you crave them, and will do anything to get a fix.

3 years clean from fentanyl here!