Cannabis is the most used illicit substance in the world, with . Nowhere is it more popular than in the US and Canada.

In 2021, aged 12 and older in the US used cannabis, roughly 1 in 5 people in this age group. Those figures have been inching upwards over the last few decades – a fact that isn’t that surprising given the expanded access to legal (or quasi-legal) cannabis and a shift in attitudes towards the drug.

The uptick in use, ease of access and increased social acceptability of the drug have some experts – and parents – worried, particularly about increased use among adolescents. That is with good reason: a growing body of evidence suggests cannabis use during adolescence , potentially increasing the risk for developing various or .

The science of cannabis

As the use of marijuana and its compounds rises around the world, 91av explores the latest research on the medical potential of cannabis, how it is grown and its environmental impact, the way cannabis affects our bodies and minds and what the marijuana of the future will look like.

Explore our coverage

Yet so far, fears of a surge in adolescent cannabis use haven’t been borne out by the data. Results from the , an annual survey conducted by the Substance Abuse and Mental Health Services Administration, showed that rates of past-month cannabis use among teens aged 12 to 17 in the US have actually declined over the past 20 years: from 8.1 per cent in 2002 to 4.8 per cent in 2021.

Instead, the increase in cannabis use is being driven entirely by adults. The 12 to 17-year-old crowd now has the lowest rates of past-month cannabis.

Some health professionals are sceptical that we are seeing the full picture when it comes to teen use, however. “I cannot believe that that is true, that it has not gone up,” says clinical psychiatrist at Columbia University in New York. “Every other piece of information would suggest it should be going up.”

For instance, we know that more people now think of cannabis as a relatively benign substance. “In general, when perceptions of things move toward safety… that increases the likelihood” of use, says Sultan. Legalisation of recreational cannabis has also been linked to increased uptick in use of the drug.

Sultan isn’t ruling out the possibility that expanded legalisation has diminished the drug’s allure, or that kids are choosing to wait until they are older to use weed. If those things turn out to be true, it would be welcome news, he says. But he thinks it is more likely there is a missing piece to the puzzle, hidden by a lag in the data collection.

As we have reported on the science of cannabis in recent weeks, nearly every researcher we spoke to lamented the yawning gap in our understanding of the drug’s effects on our health, the environment, society and much more. So why, given our long history with the drug, are there so many unanswered questions?

Research into cannabis really only started in earnest two decades ago. It picked up steam in the late 1990s and early 2000s, beginning with studies showing that marijuana could reduce nausea in people with HIV and in those undergoing chemotherapy. Since then, the field has exploded.

The science of cannabis

As the use of marijuana and its compounds rises around the world, 91av explores the latest research on the medical potential of cannabis, how it is grown and its environmental impact, the way cannabis affects our bodies and minds and what the marijuana of the future will look like.

Explore our coverage

Yet the number of published studies about cannabis pales in comparison to the trove of data we have on drugs like tobacco and alcohol.

“We don’t have foundational data on cannabis that we have for other stuff,” says at Columbia University in New York.

A big reason for this is that government regulations have made it exceedingly difficult to study the drug. In the US, for instance, researchers must obtain a special licence from the Drug Enforcement Administration, and they can only study cannabis grown at licensed facilities – prior to 2021, there was . Even in Canada, where recreational cannabis use has been legal since 2018, federal and provincial restrictions have .

While things are starting to change, we have reached a place where the availability of and hype around cannabis products has outpaced actual evidence of their effects. That has people like Sultan worried. “The whole treatment and medicinal thing I find very aggravating,” he says. We don’t know what doses are appropriate, what their long-term effects are or even what people are consuming, he says. “In fact, we know the things they’re getting are not what the label says they are. So how could you even do dosing recommendations when you don’t even have accurate ways of deciphering [what’s in it]?”

There are many fundamentals we have not yet got to grips with, from the way that the endocannabinoid system can influence our health to understanding how the cannabis products that are readily available to the public today are both produced and consumed, which can look very different to the controlled settings of most of the studies we currently rely on for insights. “Let’s find out what we need to find out so that we can do this in a thoughtful, intelligent way as opposed to what I feel like we’re doing, which is just running a natural experiment,” says Sultan.

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Cannabis is everywhere: today, the plant grows on every continent except Antarctica. But that wasn’t always the case. So how did this “weed” come to take over the world?

Its success is due in large part to us. The evidence suggests that cannabis first evolved nearly 28 million years ago on the Tibetan plateau, after splitting off from the last common ancestor it shared with the hop plant. At first, early humans may have unwittingly spread it. By clearing vegetation for settlements and heaping food scraps in waste dumps, they gave cannabis what it needed to thrive: open, sunny areas with fertile soil. That is why some scientists refer to the plant as a weedy

In time, humans came to appreciate the many uses of the cannabis plant, and it is believed to be one of the first plants we cultivated when we began farming around 12,000 years ago. The stalks could be dried to create fibres, the seeds could be eaten or used to make oil and the resin-coated bracts could have been used for their medicinal and mind-altering purposes (though is much more recent).

The plant’s utility enabled its spread and humans became the most important agent for its dispersal. Its seeds aren’t encased in tasty fruit, making them less attractive to animals, and they are round and heavy and lack wings that would let them hitch a ride on the wind.

From the Tibetan plateau, the plant spread across central and East Asia, with nomadic groups later carrying it into the Indian subcontinent, eastern Europe and the Middle East by about 2000 BC. As people began to cultivate cannabis for a broader range of uses, including its psychoactive effects, different varieties of the plant emerged. This gave rise to the two major types we know today – Cannabis sativa and Cannabis indica – though there is still debate over whether these are actually separate species or just .

By the Middle Ages, cannabis was grown across Europe, and Arab and Indian traders carried it into Africa and South-East Asia. Finally, colonial empires brought the plant to the Americas beginning in the 16th century.

The science of cannabis

As the use of marijuana and its compounds rises around the world, 91av explores the latest research on the medical potential of cannabis, how it is grown and its environmental impact, the way cannabis affects our bodies and minds and what the marijuana of the future will look like.

Explore our coverage