Is Cannabis today really much more potent than 50 years ago?
Levels of the psychoactive compound THC have been steadily rising in marijuana since the 1970s, due largely to growers selecting for more potent strains.
Weed isn’t like it used to be. The psychoactive potency of cannabis is on the rise, which means that so too are the risks of potentially harmful side effects. With a growing acceptance of the drug around the world, as well as legalisation in many countries and US states, regulators are asking one question with increasing urgency: just how much stronger is today’s cannabis?
There is no way to comprehensively assess all types of cannabis currently available either legally or illegally around the world. But the data we have suggest that today’s drug is much stronger than the weed of even a few years ago, let alone several decades prior.
In the US, some of the most extensive research on cannabis strength comes from the Potency Monitoring Program, a project led by the National Institute on Drug Abuse and carried out at the University of Mississippi. The data shows a clear trend: over the last 50 years, the average amount of tetrahydrocannabinol (THC) in cannabis – the plant’s main psychoactive component – has increased more than tenfold.
The uptick isn’t limited to the US. A study from 2020 examined marijuana potency trends in the US, the UK, the Netherlands, France, Denmark and New Zealand and found that average THC concentrations increased by 0.29 per cent each year from 1970 to 2017.
Over the years, growers have adopted increasingly sophisticated breeding techniques that have enabled them to select for more potent strains. “If you talk to someone who’s been a cannabis user… the joke that they’ll make is that you can’t get the regular stuff anymore,” says Ryan Sultan at Columbia University in New York.
Take seedless cannabis, for instance, also known as sinsemilla. These unpollinated female plants are cultivated because their buds and flowers contain a lot of THC-rich resin – seedless cannabis can contain twice as much as the regular seeded variety. In 1993, this highly potent marijuana accounted for less than 4 per cent of all cannabis samples analysed by the Potency Monitoring Program, but by 2008 nearly half of the samples were sinsemilla.
Those data come from investigating drugs seized by the police, however, which might not cover the full range of cannabis people actually consume. What’s more, the number of seizures available for study vary widely from year to year: in 1972 there were just 34, while in 1995 there were more than 3700.
Yet another factor at play is that the way we consume cannabis has been shifting. While dried flower generally remains the most common form in which people consume weed, edibles, vapes and other forms have been growing in popularity – and these can be far easier to cram full of THC. Some concentrates have THC levels of up to 90 per cent, for instance.
The availability of highly potent marijuana – coupled with data showing that more and more people think cannabis is relatively harmless – is giving some researchers pause. Higher THC levels raise the risk of unpleasant or dangerous side effects like nausea, vomiting, paranoia and abnormal heart rhythms.
People point to the fact that humans have been smoking cannabis for thousands of years as evidence of its safety, says Sultan. But the strains available today can be so much stronger that we are in uncharted territory, he says.
