Before injection, PWID usually filter their mix to remove certain unwanted elements and prevent their needles from clogging.
The Apothicom Blog
Injection drug use exposes to different types of complications:
Viral infections (HIV, HCV, etc), transmitted when sharing paraphernalia
Bacterial...
The product to be injected -whether in powder, solid or tablet forme, must first be dissolved in water. It may also need to be heated to facilitate dissolution.