A scientific study into cannabis strength revealed high-potency varieties of the drug were more dominant in the UK in 2016 than eight or thirteen years previously.

Stronger strains of cannabis made up 94% of police seizures in 2016, compared to 85% in 2008 and 51% in 2005.

This is based on data from the same areas for the three studies – London, Kent, Derbyshire, Merseyside and Sussex.

The study showed that the dominance of high-potency cannabis strains in the UK were due to the reduction in the availability of weaker strains.

Accessibility of weaker cannabis resin, which is what creates a less strong product, declined from 43% in 2005 to 14% in 2008 and 6% in 2016.

The study was undertaken by researchers from King’s College London’s (KCL) Institute of Psychiatry, Pyschology and Neurosciene and Drug Control Centre in collaboration with scientists from GW Pharmaceuticals based in Cambridge.

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By GlobalData

KCL and GW Pharamaceuticals’ research was published yesterday in Drug Testing and Analysis.

What this means:

The study highlights the increased threat to users’ mental health as result of high-potency cannabis varieties dominating the UK drug market.

Senior author of the study, Dr Marta Di Forti, MRC Clinician Scientist at KCL’s Institute of Psychiatry, Psychology and Neuroscience said:

“In previous research we have shown that regular users of high-potency cannabis carry the highest risk for psychotic disorders, compared to those who have never used cannabis.”

“The increase of high-potency cannabis on the streets poses a significant hazard to users’ mental health, and reduces their ability to choose more benign types.”

She continued:

“More attention, effort and funding should be given to public education on the different types of street cannabis and their potential hazards.”

“Public education is the most powerful tool to succeed in primary prevention, as the work done on tobacco has proven.”

KCL’s Institute of Psychiatry, Psychology and Neuroscience published a study in 2016 that confirmed the 2018 study’s conclusion on a global, rather than UK only, scale.

It showed that cannabis use is associated with an increased risk of psychotic effects. High-potency and synthetic cannabis strains are the most likely to lead to negative psychotic outcomes.

This study concluded that there was not conclusive evident that cannabis use increase risk of psychiatric disorders, but it is certain that cannabis use has a negative effect on users’ cognitive function.


Cannabis remains the most widely used illegal drug in the UK, according to Home Office statistics on drug use for 2015/6.

It’s thought 6.5% adults aged between 16 and 59 having used it in the last year. This is approximately 2.1 million people.

It is especially popular among young adults. 15.8% of adults aged between 16 and 24 used the drug in the last year. This is around 975,000 people.

These figures are lower than previous years.

In 2005/6, the percentage of adults between 16 and 59 using cannabis was 8.7% and of adults between 16 and 24 was 21.4%.

The UN Office on Drugs and Crime data was used by the Telegraph newspaper to map the world according to cannabis consumption. The UK ranked 26th in the world.

Iceland ranked highest with 18.35% of the population using cannabis. Followed by the US, Nigeria and Canada.

Both the US and Canada have moved towards legalisation of the drug in the past few years.