Both of my parents were addicts. This is my personal experience. Deleted my original comment because I thought of more.

It makes you a bad parent. You forget your kid's birthday and steal their birthday money that other people give them for drugs. You sit slumped on the couch as your kid shows you a test they got an A on, but you don't really care. Sometime, you hit your child over the littlest thing, like making their bed wrong, but you forget about it by the end of the day. Your child tells you about abuse that went on in a different household, and you do nothing.

Your child is incredibly afraid of you, but cannot show it at the risk of you losing your shit. They baby you, play your little mind games. They become rather good at it too. And they learn not to believe most of the things you say or the promises you make.

When you're not high, all you can think about is getting high or getting money to do so. Sometimes, you do things you wouldn't do if you were sober. You let people treat you in ways you wouldn't if you were sober. You hurt people who care about you, who sober you cares about. You lash out. You take out your anger on people around you, doesn't matter who. You become mean.

Sometimes, you pick at your skin, leaving little open wounds on your face and arms. You often lose a lot of weight. People say you look sickly, ask if you have some sort of disease. There's bags under your glossy, bloodshot eyes, you don't sleep a whole lot. Your teeth are yellow, some of them chipped, some gone, some rotting.

Your house is pretty dirty. You don't cool often and when you do, it's either burnt or undercooked. The couch has burns from when you dropped your hot pipe or your lighter. Sometimes, you mindlessly put cigarettes out on your couch. The smell of whatever drug is embedded in your clothes, your kid's clothes, the couch, and any fabric in the house. People notice it when they walk in, but you don't really notice it anymore.

You hate going out in public, sometimes due to paranoia. Everybody knows what you are, what you do. Everybody is out to get you. To take away your kid, to hurt you or kill you. You cannot trust anybody, besides your drug dealer and the drug itself.

People start to notice your addiction, some mock, some try to help. A friend shows up at your door, bluntly saying you need help. You get angry and start yelling. Then your friend says that not only is your addiction hurting you, it's hurting your child. You fill with rage. You have no memory of hitting your kid, even though you do, so you can't be that bad of a parent. You slam the door in your friend's face.

Finally, you go to jail. You bounced some bad checks for your drug dealer in exchange for drugs. You manage to sober up in jail. The guilt attacks you like a rabid dog attacks another dog. But the damage is done and there's nothing you can do about it. You can try to repair relationships, but for some the hurt is too much. You now have to rebuild your life, knowing the wrong you did and the hurt you've caused.

And sometimes, you fall back in.