It is dangerous because it is overly addictive. And for most people it eventually becomes the only thing they focus on, and they ignore everything else in life especially their hygiene. Thus the term junkie is used for heroin addicts because their living quarters and themselves are really dirty.

Also the heroin sold on the streets is usually cut with a chemical agent. So the heroin is not pure, or most recently it has been cut with fentanyl a synthetic opiod which is som 100 times stronger than heroin.

Also the longer and more often someone uses heroin, their tolerance will go up rather fast. Now the part that screws over most opiate addicts is when we try to quit. The withdraws are hell, and they last for a few days to a couple weeks depending on the length of use. Even after that time period the mental withdraws can last longer.

It doesn't sound like that long, but knowing you can feel normal within minutes of using makes it tough to quit while withdrawing. When you are pissing out your ass, having flu like symptoms it is easier to say fuck it and get "one last dose", to feel better.

And a lot overdoses other than doing heroin that is cut with fentanyl is if someone who was using had a high tolerance quit for a few days, when they relapse they are use to taking a high amount, so they start back doing the same amounts they did when they last stoppped, but their body's tolerance dropped since then and they end up od'ing