Hi. I donated a kidney, but I have read about liver donors for some years now as well. You should, of course, answer honestly all questions asked. But they check ALL livers to make sure they are functioning well and have no problems before they approve a donation, not just those who may have a special situation. Regarding past drug use they are more concerned if the drug use involved needles, which could result in infection (HIV, Hepatitis, etc.) But as far as I know you would not automatically be ruled out.
Fr. Pat