Helping kids avoid opioid addiction

How can I make sure my kids don't get caught up in the opioid epidemic? I've been reading about it, and I'm worried. These particular drugs – especially the prescription varieties – seem to be widely available and easily accessible, even on school campuses. What I really need to know is why teens and preteens start (and continue) using drugs. And what steps can I take to protect my children against the dangers of substance abuse?