system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do addicts veins collapse from using injectable drugs?

There was a previous question related to this, but there were only a handful of answers and they were contradictory, and no one identified themselves as a reliable authority. 

So, what happens?

EDIT: thanks everyone for the great answers.
gpt
There are multiple factors to this:

*  The veins have to heal after every puncture. If they use drugs a daily or multiple times a day, it becomes too much. It will become fragile and it can burst.
* The drugs they use hurt the inner lining of the veins. In medicine, there are guidelines to what you can put into a vein, how much do you have to dilute it and how fast you can give it. Drug users will not care about these things.
* The inner lining of the vein is what keeps blood from clotting. If it is damaged, blood will clot and the vein will collapse.