Talk to your kids about the dangers of sharing needles and what infectious diseases that can lead to. Of course, outside of any prescriptions they have, they should never be injecting themselves with any drugs on their own, but this is a good lesson in the other terrible consequences that could happen if they do.
There was a story on npr.org about an HIV outbreak in southern Indiana due to the sharing of needles and the abuse of the painkiller Opana. This story talks about the reason why people started injecting the drug and the outbreak in Indiana. It has some pretty staggering statistics on how many people were infected in this outbreak and is an excellent example of why not to use needles.