That's fairly easy to do...look at how many addicts throw their lives away screwing over their families in the process.
Look at how many people are killed by them.
Look at the accidents caused by drivers who are under the influence.
Drugs never hurt anyone but the user, what a load of utter crap.
The idea that drugs only harm the user is a bit silly, of course.
But a question is whether prohibiting drugs is an effective way of stopping that harm. All those bad things happen currently - what we need to ask is could some of it be reduced by legalisation, or would it be increased?
Deaths from using drugs may be reduced by cleaner supply, more point of sale usage guidelines, and a wider understanding of the effects/risks.
Unsafe behaviour, such as drug driving, can be reduced through education (Drink drive deaths have roughly halved every 10 years since the '70s, for example). And there aren't all that many anyway (47 drug drive deaths in 2014).
Families do suffer, though alcohol is a wider problem in this regard. Perhaps legalisation could bring more support services, and reduce (through lower prices, cleaner supply, and less risky supply) the negative associated behaviours. Young women being trapped by drugs into sex work, for example - often leading to chaotic lives involving pregnancies and the spin-off problems there.
I get drug tested in my line of work.