Clay, I work with teenage boys with problem behaviour, most of whom use drugs. It is very hard to get them to engage in worthwhile discussion about the potential risks, esp of weed. My approach is to literally & figuratively kick them where it hurts - in the nuts!
This approach uses the fact that they revel in their status as rebels, both against the law and against society. The easiest way to break the law is through drugs; the easiest way to attack society is through sex (When a 15 year old boy shags a 13 year old girl, he is giving 2 fingers to his school (remember PHSE?), the law, both families, lots of social workers, the Daily Mail etc...). I aim to neutralise these impulses by putting them in conflict with each other.
I explain to them that the cannabinoids (the chemicals that give them the buzz from weed) are predominantly derived from female plant hormones, and that these have the effect of lessening their interest in sex. I have a folder of academic articles on the subject: none of them will ever want to read the full articles, but the abstracts can be pretty stark. For instance,
www.ncbi.nlm.nih.gov/pubmed/19733173
If necessary, I reinforce this by asking them if they have had / ever wanted to have sex immediately after smoking a joint. The answer is invariably "No": although this is more as a result of the hallucinogenic effects of the drug, rather than its transitory caponising effect, it nevertheless drives whom the point: weed makes you less interested in sex.
There are a couple of consequent effects of this that I generally ignore:
I don't labour the point that heavy long-term use reduces fertility in case any of them think of it as a form of male contraception, and use that as an excuse to not use proper prophylaxis.
I forget to tell the boys that the female plant-hormone effect increases interest in sex in girls: a former bf of mine identified this effect & called it gajgaj (Give a joint, get a jump!). For instance, for the article I ref above, I show the boys a copy of the abstract with only
"...suggesting predominantly detrimental effects on male sexual motivation and erectile functioning."
highlit (in neon pink). For some reason, they never read the rest of the abstract, let alone the whole article.
The point I try to make is that weed is a drug that has both benefits and costs, and that they as would-be users need to make their own analysis of these. In general, once they carry out a proper analysis (often informally), they decide that the cool factor of using weed cannot compete with wow factor of being a sexy beast, so their use declines.