Is it the parent's fault if their child becomes an addict? Sometimes addicts come from wonderfully well rounded and supportive families, sometimes not. There is a myth that all addicts have bad parents. Most intelligent people realize that's wrong - addiction does not discriminate and no one is immune. If you are a parent who truly did the best you could and your child still became addicted, then this post isn't for you. This post is for the rest of the parents, those who can share the blame, intentionally or not for their child's addiction.
If you were one of those who did drink and dope when your kids were young and then they followed in your footsteps, please apologize. If you neglected them or allowed them to be abused, please apologize. My father and I are not very close, but one of the things I will always remember was his apology a few years ago for his behavior and drug use when I was young. It wasn't long and heartfelt. It was just a simple "I'm sorry that I did all that" and I felt like he meant it. I thought I had moved on already, but those simple words were more healing than I could have imagined.
Your child may not be in a place yet where they can or are willing to accept your apology, but when they get there knowing you said it and were sincere will help with their healing and most likely yours too.