Is arm amputation necessary sometimes after intravenous drug use?

Sometimes: If an infection sets in amputation may be necessary to save the person's life. If clots are present it might b e needed too.

Yes: Sometimes the addict accidentally injects an artery t hereby causing it to clot. This can yield immediate severe pain and eventual gangrene. Sadly, there is no way to reverse this damage -- no operation, no medicine, no therapy. The only thing to be decided is how much is amputated -- fingers, hand, wrist or forearm.