The thing about drugs of abuse is that they make you feel good. If they didn't make you feel good nobody would ever take them. Nobody becomes an addict by using a drug for one time. The problem starts when the user likes the effects the drug has on them and seeks to repeat that experience. Repetition causes two further problems: Firstly, tolerance, i.e. users need a higher dose in order for the same effect to be observed. For instance someone who does not drink alcohol regurarly may get drunk with a couple of pints of beer, whereas an experienced drinker needs for example 7 pints to get as drunk as the non-experienced drinker. When the dose of the drug gets too high, it starts affecting other organs as well as the primary site of action, and therefore other secondary side-effects can be observed. In cases of some drugs the user will eventually die of OD. The second problem is physical dependance, i.e. the user's organism needs the drug in order to function properly. Is the user is deprived of the drug, then several side-effects can be observed. These side-effects for some drugs (e.g. alcohol) can be lethal. The user needs to take the drug regularly in order to prevent the side-effects from occuring. This leads to tolerance etc.
Overall, once you mess up with drugs-you're fucked.
(Quoting South Park) "Drugs are baaaad mmmmkey?"
Take care...