My sister has three children, two of which were brought up by my mum, the third boy was kept by her for the first four or five years (that was when she wasn't abandoning him at my mums, saying she was going out for the night and not returning for days,weeks or months!)
The first two children,two boys, are well adjusted, hard working lads in their early 20's.
The youngest boy has a lot of problems. He has been drinking since about the age of 12 and we suspect he is smoking cannabis and has done for a while.
He has been in trouble with the police from an early age (also about 12yrs old) and does not have any respect for authority.
What is very distressing though is the fact that we all know he loves his mum, but has had the most vicious arguements with her and has even physically fought with her!
She pretends to understand why he is this way, but then she starts on about him being a 'bad lad', so no, I truly dont think she understands what she has done to him.
The surprising thing is though, I do not think all drug addict parents are a automatically going to be 'bad parents'.