It most assuredly can be, however, there's two ways to get the hair back, one is to stop using, and the other is propecia (works wonders).
It can be. Methadone use long term can cause hair loss. I'd assume it's probably the same for other opiates. I believe it has something to do with certain drugs inhibiting calcium production, among other things.
Taking calcium tablets will only help so much, as the drugs block the actual body's ability to process and direct calcium properly.
I also remember potassium is a problem too (lack of due to drug use) but I'm not sure if that plays into hairloss!