Data coming out of Washington and Colorado suggest that those states' legalization experiments, which began in earnest in 2014, are not causing any spike in use among teenagers. Teen marijuana use in Colorado decreased during 2014 and 2015, the most recent time period included in federal surveys. A separate survey run by the state showed rates of use among teenagers flat from 2013 to 2015, and down since 2011. A state-run survey of 37,000 middle and high school students in Washington state finds that marijuana legalization there has had no effect on youngsters' propensity to use the drug. The Washington State Healthy Youth Survey found that the 2016 rate of marijuana use was basically unchanged since 2012.

Teen marijuana use fell sharply in Colorado in the years 2014 and 2015, after the opening of that state's recreational marijuana market, new federal survey data show. The state-level data from the National Survey on Drug Use and Health showed that 18.35 percent of Coloradans ages 12 to 17 had used marijuana in the past year in 2014 or 2015, down sharply from 20.81 percent in 2013/2014. That works out to roughly a 12 percent drop in marijuana use, year-over-year. Year-over-year teen marijuana use fell in most states during that time period, including in Washington, the other state to open recreational marijuana markets in 2014. But that drop wasn't statistically significant.

Canada became the second country to make it legal for adults to buy, grow and consume small amounts of marijuana. But it also made it a crime to give it to anyone younger than 19 or 18, depending on the province, and set a penalty of up to 14 years in prison for doing so. At the same time, the government began an $83 million public education campaign, much of it targeting Canadian youths, that warns of pot’s dangers. But persuading teenagers not to see legalization as a green light to use marijuana will be difficult, experts say, not to mention that past antidrug efforts have offered little evidence of success. And when it comes to marijuana and the teenage brain, the science is far from clear.

Before the new law came into force in October 2018, Statistics Canada started to estimate prices and the size of the illicit market, and to carry out quarterly surveys of Canadians’ cannabis usage. Earlier this month it released the fifth of these—the first before-and-after comparison of the same part of a year. The main finding was a rise in the number of Canadians who had used cannabis in the three months before the survey, of 27% compared with a year earlier. People are probably more willing to admit to getting lit once weed has been legalised. However, half of new cannabis users are aged over 45; use by under-25s, by contrast, did not rise significantly. Nor was there a significant increase in the number of Canadians who said they used daily or near-daily.

Fears that cannabis causes irreparable harm to teenager’s brains have been stoked by trials which “overstated” the effects on intelligence and other functions, according to a review which found little ill-effect after three days abstinence. Studies have shown it is 114 times less harmful than alcohol, but marijuana’s impact on adolescents’ brain development and mental health is a major concern for policy makers in debates over legalisation. This is a key time developmental period and studies have found negative impacts on attention, learning, memory and organisation in heavy or frequent cannabis users. The study found that the “persistence and magnitude of impact” on teenagers had been overblown.

Scientists believe they have identified about 60,000 cases of depression in adults under 35 in the UK, and more than 400,000 in the US, that could be avoided if adolescents did not smoke cannabis. An international team of scientists looked at 11 studies published from the mid-1990s onwards, involving a total of more than 23,000 people, they report in the journal JAMA Psychiatry. They explored the use of cannabis for non-medicinal purposes in under-18s. Participants were then followed into adulthood to see who developed clinical depression, anxiety or suicidal behaviour. No single study looked at all three mental health issues. (See also: Teenage cannabis use linked to depression in later life)

Rates of cannabis use disorder have risen in U.S. states where the drug has been legalized, including among children and teenagers, according to a study. The authors of the research published in the journal JAMA Psychiatryargued that while the policies have provided "important social benefits, particularly around issues of equity in criminal justice," the climb in conditions like cannabis use disorder are "a potential public health concern." "Given our findings on problematic use across age groups, legalization efforts should coincide with prevention and treatment." (See also: A new study found marijuana legalization leads to more problematic use |Study finds declining trend in prevalence of cannabis use disorder among frequent users)

The Danish government will present a bill to make it much harder for young people to abuse laughing gas cartridges to get high. The bill, which has a broad majority, will make it illegal for under-18s to buy the nitrous oxide cartridges, which are most commonly used in siphons to produce whipped cream for coffee and desserts. Additionally, it will be illegal for anyone to buy more than two of the eight-gram cartridges at the same time. The cartridges have been popping up all over cycle lanes and pavements over the last two to three years due to their abuse by teenagers who inhale the gas for a short-term euphoric effect. (See also: Why Denmark wants to ban under-18s from buying laughing gas)

Consuming cannabis every day could delay at-risk youth from moving on to injecting more dangerous drugs, according to a new study that casts doubt upon the age-old assumption that marijuana acts as a gateway for teens to try other more harmful substances. The research, from scientists at the BC Centre on Substance Use, also adds to other work that has suggested marijuana could be used as a substitute for people addicted to opioids. Researchers repeatedly interviewed 481 homeless young people in Vancouver’s downtown core who had never injected any drugs and found - over a decade of tracking this at-risk cohort - that daily cannabis use was associated with a 34 per cent decrease in the rate people started injecting drugs.

Late last year, the press and marijuana-legalization opponents gave a lot of attention to a studysuggesting that daily marijuana use shrinks users' brains. New research indicates that claim and other reports of cannabis-caused changes to brain structure simply aren't true. The new study, "Daily Marijuana Use Is Not Associated with Brain Morphometric Measures in Adolescents or Adults," published in the latest edition of the Journal of Neuroscience, suggest that alcohol use was responsible for previous studies finding brain changes.

Heavy marijuana use is associated with cognitive decline in about 5% of teens, according to a new study, which suggests that the heaviest users could lose 8 IQ points, according to a report, published in the Proceedings of the National Academy of Sciences.If the link is real, the effects on cognition could be dramatic. But intelligence and cognition is affected by a plethora of other factors, including genetic, social and environmental influences that may supersede any influence from drug use. 

The relentless crackdown by security forces on the mainly cannabis-smoking youth in Beirut has had several negative repercussions on the Lebanese society. Young, impressionable teenagers in Beirut are increasingly getting drawn to what is called "synthetic cannabis" or otherwise known as "K2" or "spice." A mixture of herbs is usually laced with cannabinoids such as cannabicyclohexanol. The exact effects of this mixture are still not well understood, but early studies suggest a severe increase in chances of psychosis.

Despite concerns that legalizing weed would lead to all hell breaking loose, most things have remained the same—and teens are reporting consuming less weed than before, according to a new report from Statistics Canada.Stats Can has been conducting its National Cannabis Survey, an online questionnaire, since February 2018 in an effort to collect data on Canadians’ cannabis habits before and after legalization. While the data has limitations—it’s based on self-reported surveys and answers haven’t been verified—it does paint a pretty chill post-legalization picture. According to Stats Canada, the rate of cannabis consumption for 15-17 year olds fell from 19.8 percent in 2018 to 10.4 percent in 2019.

Following legalization, the rate of adolescent marijuana use in Colorado has fallen to its lowest level in nearly a decade, according to new federal survey data. State-level numbers from the National Survey on Drug Use and Health show that a little more than 9 percent of Colorado teens age 12 to 17 used marijuana monthly in 2015 and 2016, a statistically significant drop from the prior period. That's the lowest rate of monthly marijuana use in the state since 2007 and 2008. And it's not just marijuana: Rates of teen alcohol, tobacco and heroin use are down sharply in the state, as well.

France and Canada have the highest percentage of 15-year-old cannabis smokers among 42 well-off nations surveyed by the World Health Organization. When it comes to policing marijuana, France is far from the most laid-back country in Europe, so the findings come as something of a surprise. Amsterdam flaunts its cannabis cafes and Barcelona its private reefer clubs, but neither the Netherlands nor Spain were among the top eight nations in which teens admitted they had used cannabis in the last 30 days, according to the study. France dislodged Canada as No. 1 nation for teen tokers

Prohibitionists warn that it’s dangerous even to discuss legalizing marijuana because such talk sends “the wrong message” to the youth of America, encouraging them to smoke pot. If so, you might expect that the legalization of marijuana in Colorado and Washington, approved by voters more than a year ago, would have a noticeable impact on marijuana use by teenagers. Yet the latest data from the government-sponsored Monitoring the Future Study indicate that teenagers continued smoking pot at pretty much the same rates as before.

Japan has seen a sharp increase in marijuana possession arrests, especially among teenagers and people in their 20s, prompting warnings of drug-related issues typically associated with the more tolerant West. But the number remains relatively low for a country of more than 127 million people. National Police Agency figures show 3,008 people were arrested on marijuana changes in 2017, up almost 20 per cent from 2,536 cases in 2016. The spike marks a new record for the largely drug-intolerant country and comes as arrests for hallucinogenic substances are declining – apparently due to a police crackdown on “dangerous drugs”.

Trudeau's argument for legalization is concerned less with creating benefits, and more with reducing harms. He starts from the same place that many legalization opponents start from — concern for the safety of children. He points to an easy-to-overlook fact: It's alreadyincredibly easy for teenagers to get high if they want to. In 2015, for instance, nearly 80 percent of U.S. 12th-graders said it would be easy for them to obtain marijuana. It's clear, in other words, that current policies centered on making the drug completely illegal are doing little to keep it out of the hands of kids who want to use it.

New research highlights associations between teenage cannabis use and a range of mental health problems. The results suggested that use of the drug was associated with an increased risk of depression and a significantly higher risk of suicide attempts. As usual in a study based on survey data, the authors noted that a clear line of causation from cannabis use to the reported effects cannot be drawn. There are always other potential mechanisms in action. Young people who use cannabis regularly may already be experiencing mental health issues that make drug use more likely; or be facing adverse life experiences that influence both their mental health and drug consumption. (See also: The government should proceed with caution when it comes to cannabis)