Nothing. There has been no spike in drug use since legalization and decriminalization efforts spread across the country in recent years. Despite all the fire and brimstone anti-marijuana advocates yell down, teens aren't smoking more pot.
According to the CDC's Youth Risk Behavior Surveillance System (YRBS), the number of youth who have tried marijuana has even dipped slightly over the last decade from 42.4% in 2001 to 40.7% in 2013. The number of current users (who consumed weed in the 30 days before the survey) remained steady at about 25% of all teens.
This news debunks many arguments against legalization, most of which assert that legalizing pot would cause a surge in marijuana consumption, particularly in teens. Just before legal sales became a reality in Colorado at the start of this year, the National Institutes of Health (NIH) reported that fewer teens viewed pot as a harmful drug and were more willing to try it. Their study found that even the good kids — the religious ones and those who didn't drink or use tobacco — were easing their views on pot, threatening to increase consumption among America's high schoolers.
The wave of legalization has fostered a lot more media and information on marijuana than ever before, but apparently young people aren't taking that as a green light. They're not going crazy with weed just because they are more aware of it, nor because it's becoming easier to access in several states. Despite the cries of "Won't someone think of the children!?" it seems the children are thinking for themselves just fine.
In fact, drug use is on the decline among youth. Based on the YRBS' findings on the use of other drugs among teens, intoxication in general is not as popular as it once was. Since 2001, about half as many kids have tried cocaine, ecstasy or other kinds of hallucinogens. Heroin use stayed the same at 2.2% of youths, but meth experimentation is down to 3.2%, nearly one-third of its 9.8% frequency in 2001 (statistics on meth don't include the legal methamphetamine in prescription drugs like Adderall and Ritalin). The abuse rate of prescription drugs is down about 3% since 2001 and currently sits at 17.8% (again, this number does not include kids who have prescriptions). But the 11% of young people ages 4-17 diagnosed with ADHD (up from 7.8% in 2003) can take that speed legally and it's not called abuse.
But opponents still harp on the perceived qualitative effects of legalization. They paint it as a threat to children, an effective emotional argument rooted in 1930s era "Reefer Madness," which spawned the namesake film, alternately titled Tell Your Children or sometimes Doped Youth. Because high school kids are now more aware of weed, the assumption was that they would be more likely to abuse it. This thinking discredits the modern American teenager's ability to make informed personal decisions. If kids are modifying their views on pot based on the information available, they're not automatically going to start abusing it. Now, we have the numbers to prove it.