We should disallow everything which could cause the slightest problem in humanities aim to live only for productivity. God forbid that humans also want to do stuff which they like and don't only think about how they could be as productive as possible.
Those movies about the future where people are only allowed to do what makes every single person as effective as possible remind me about this thread.
All I can say about my personal experience is that everyone who wants to take this already does it (which were probably more than 50% of the people I knew when I was younger) and of all these people, nobody ended as someone living in the Metro pumping some worse stuff in their veins. Most of them also stopped with it when they were 25 or so so that it doesn't seem to be too bad. All those people who drank alcohol still do it 
The only real difference between allowing it and disallowing it is that criminal "associations" profit a lot from the fact that is isn't allowed since that is the reason why people get it from them.
And to be honest, I think games are also a problem for a few people so that we would also have to disallow them if you think "making people lazy" is a problem for the society.
