If we classify drugs by the effects they produce, then there are 3 types of drugs:
You don’t get addicted from one day to the next. It can take months or years. It starts with the occasional use of a substance. Some people stick to that level, but others get carried away and begin taking the substance more and more. That happens especially if you start using alcohol or drugs to get out of a bad mood: you’re feeling fed up or stressed out, but a few drinks or joints make you feel better. Obviously that’s not going to help for long, and the bad mood comes back the next day. So you keep trying it again and again, and you need the substance more and more often. And otherwise you start craving it. You’ve grown psychologically dependent on it.
Not only that, but your body gets more and more accustomed to the alcohol or drugs. At some point, they don’t help anymore and you need increasing amounts to get the same effect as before. You start taking higher doses. Your body now expects to receive alcohol or drugs every day. If you stop or cut back, your body protests. You may even get sick. That is called withdrawal symptoms. That is physical addiction.