Cannabis now twice as strong as it was 11 years ago, study says

Cannabis strength has doubled across Europe in the last 11 years, according to the first study to track changes in the drug across the continent.

Researchers from the University of Bath and King’s College London found both cannabis resin and herbal cannabis have increased in strength and price – with potentially harmful effects for users.

The research, published in the journal Addiction, used data collected from across 28 EU member states as well as Norway and Turkey, for the European Monitoring Centre for Drugs and Drug Addiction.

In herbal cannabis, concentrations of THC – the main psychoactive constituent of cannabis which has been linked to psychosis – increased from 5 per cent in 2006 to 10 per cent in 2016.

For cannabis resin, THC concentration rose from 8 per cent in 2006 to 10 per cent in 2011, but increased to 17 per cent by 2017.

The price of herbal cannabis jumped from €7.36 per gram to €12.22 (£11) between 2006 and 2016 while the price of cannabis resin increased from €8.21 per gram to €12.27 per gram during the same time period.

Dr Tom Freeman, lead author of the study, said: “These findings show that cannabis resin has changed rapidly across Europe, resulting in a more potent and better value product.”

But speaking of the potential side effects Dr Freeman, from the Addiction and Mental Health Group within the Department of Psychology at the University of Bath, added: “Cannabidiol (CBD) has the potential to make cannabis safer, without limiting the positive effects users seek. What we are seeing in Europe is an increase in THC and either stable or decreasing levels of CBD, potentially making cannabis more harmful.”

Cannabis resin typically contains CBD as well as THC. CBD may offset some of the damaging effects of THC such as paranoia and memory impairment.

New resin production techniques in Morocco and Europe have boosted levels of THC but not CBD.

In Britain alone, THC levels in herbal cannabis remained roughly similar between 2006 and 2016, but police seizures indicate they have increased steeply in cannabis resin.

Cannabis is used by an estimated 192 million people each year worldwide in a range of drug markets – spanning from areas under heavily sanctioned prohibition to those where commercialised sales are legal.

Recreational use has now been legalised in Canada and a number of American states, while a law change in November allows UK patients to get medicinal cannabis if prescribed by a specialist doctor.

Earlier this month, a study suggested that cannabis users whose first experience with the drug is with potent “skunk” strains are nearly five times more likely to go on to display signs of dependence.

US researchers discovered higher concentration of THC increased the likelihood that users would develop cravings or risky drug use that unsettles their daily life.

Although medical or recreational cannabis use has been legalised in a growing number of places, there is little regulation on potency – even though this is routine for drugs like alcohol which also cause impairment and dependence.

Dr Brooke Arterberry, an assistant professor of psychology at Iowa State University who led the research into THC and dependency, said: “THC has linearly increased over two decades.

“Based on the results, states may want to think about the available potency levels of cannabis products, especially with the changing legal landscape of cannabis.”

High-potency skunk has pushed out virtually all other varieties in the UK – with experts warning mental health disorders like psychosis are more common in people who use these stronger strains.