It's technically known as Desomorphine or "Krocodil". It's basically a cheap and badly made morphine substitute. Hence the toxins come from the drug itself rather than the needle, although dirty needles don't help, of course.
And yes, Necrotising fasciitis is common. Most users of "Krocodil" have a 2-3 year life span. That's much higher than the average junkie btw.
Here are the after affects. just cos it's on Youtube don't be fooled. THIS IS GRAPHIC. You have been warned......
In fact I won't post it here. I feel this is the wrong place to do so. If like me you are curious then search for "VERY EXPLICIT Man's rotten leg sawn off" on youtube.
Enough said...DON'T INJECT THINGS INTO YOURSELF!