I was floating around youtube and I found this interesting video. This man explains the addicted brain. I found this video very informative. It talks about replacement therapies too, but the science behind why we keep using is something I thought you might all be interested in :)
http://www.youtube.com/watch?v=udhE338yaOM&feature=related (part 1)
http://www.youtube.com/watch?v=jchbkA5Rofw&feature=related (part 2)
WannaBeFree