The demise of a person's character with drug addiction - have you been a witness to how drugs take over?

Can anyone share their experiences of losing someone they love to an addiction - preferably substance abuse?? With the term "lose" that does not necessarily have to imply that they have died. I'm just curious as to how a person's personality changes, their mental state & functioning, their decision making. Losing sight of people & things that were once important. The hold that drugs seem to have on a person - owning them and/or taking their souls. Answers are appreciated from family/friends of addicts or addicts themselves. I am also interested in triggers. Can situations trigger episodes or relapses, or are situations used or manipulated to the advantage of addicts, to give them a reason to use?