Much of what you've read in the last few pages described the dangers of either regularly abusing drugs or alcohol, or becoming dependent on them - that is, of developing some kind of problem with these substances.
You may be thinking that you know of people who get drunk now and then - or smoke cannabis occasionally - and they don't appear to have any problems with their drug use.

Well, we'd warn anyone who believes that casual drug abuse doesn't pose any danger, that in our experience both drug abuse and drug dependence are progressive.
They start small . . and get bigger . . and bigger . . and bigger

It's very easy to move from cannabis to amphetamines to heroin - or from a weekly drink to a daily drink to permanently drunk - i.e. taking more powerful drugs or taking more of a single drug. Remember, this can develop slowly - over some years.

Would you like a so-called 'casual drug user' to be your taxi or train driver? To look after you in hospital? To do electrical wiring in your home? To repair the brakes on your car? To be driving towards you just as you cross the road?

There's no such thing as a safe level of drug abuse.