I would have concerns as well. The teenage brain is subject to poor judgment, even in the best of kids. My approach, right or wrong, has been to talk about the short- and long-term effects of drug/alcohol use early and often. I am prepared to say "it's not that I don't trust YOU, it's that I don't trust the situation"; if a kid is in an environment where there is a high probability that others around them are impaired, whether or not s/he partakes, s/he is still at risk.
This report on marijuana was just released today. I have not read it yet but thought I would provide the link.
http://www.rmhidta.org/html/2015%20FINAL%20LEGALIZATION%20OF%20MARIJUANA%20IN%20COLORADO%20THE%20IMPACT.pdf
Here is a short blurb about impaired driving:
Section 1 ? Impaired Driving:
? In 2014, when retail marijuana businesses began operating, there was a 32
percent increase in marijuana-related traffic deaths in just one year from 2013.
? Colorado marijuana-related traffic deaths increased 92 percent from 2010 ? 2014.
During the same time period all traffic deaths only increased 8 percent.
? Marijuana-related traffic deaths were approximately 20 percent of all traffic
deaths in 2014 compared to half that (10 percent) just five years ago.
? In 2014, when retail marijuana businesses began operating, toxicology reports with positive marijuana results of active THC results for primarily driving under the influence have increased 45 percent in just one year.
abl