I am serious. I don't have links. I have 20+ years of experience of dealing w/ people like that and all the times I've looked it up over the years. My store caters to them. I have first hand knowledge. It's been my joy and pleasure, that would be sarcasm, to watch the varying effects of varying chemicals on various people over various amounts of time. I can tell you what happens when someone becomes a crackhead, meth junkie, cokehead, speed freak or starts taking hallucinogens. Any hallucinogens.
Seriously, if you want to keep a kid off of hard drugs, sit 'em in my store for a week and tell 'em to just watch and listen.

While any individual may react in different ways to different chemicals, there are some themes common to all who use a specific chemical. One thing that seems common to all users of any hard drug is that they like to talk. And talk with one once a week or so for a year or so and you'll be able to see the degeneration. Talk to many of them for long enough and you'll see patterns of what certain chemicals do to the mentality of their users.

With that in mind, consider the brain. It is a fantastically complex biochemical computer. Biochemical. I'm pretty sure if one does a little reaserch, one will see there are all kinds of dangers in altering that biochemistry artifically and with out a medical necessity. Actually, even with a medical ncessity there's all kind of dangers.
While I am loathe to take anyone's word for the harmful effects of drugs because of the lies told about pot, coffee, tobacco, and alcohol, I can vouch for the negative effects of hard drugs reported by various institutions and government agencies. I've seen it first hand. In some cases, over a period of years.

Sometimes when you think everyone else has a problem, the real problem lies within. Okay Mr. I'm-better-than-you?