Cannabis resin and herbal cannabis have doubled in potency in a decade, according to a pan-European study.

Concentrations of THC in herbal cannabis - the main psychoactive element of the drug - increased from 5% in 2006 to 10% in 2016.

For cannabis resin, it rose from 8% in 2006 to 10% in 2011, then jumped to 17% by 2017.

The research for the European Monitoring Centre for Drugs and Drug Addiction used data from all 28 EU countries, and also Norway and Turkey.

The price of the drug also increased.


Herbal cannabis went from €7.36 per gram in 2006 to €12.22 in 2016.

Cannabis resin rose from €8.21 per gram to €12.27 over the same period.

The study's lead author, Dr Tom Freeman, from the University of Bath, said the findings showed cannabis resin had changed across Europe to make it a "more potent and better value product" - but with higher potential risks.

In addition to THC, resin normally contains cannabidiol (CBD) - which may offset some of the effects of THC such as memory problems and paranoia.

New resin techniques in Europe and Morocco have increased THC, but not CBD.

Dr Freeman said: "CBD has the potential to make cannabis safer, without limiting the positive effects users seek.

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"What we are seeing in Europe is an increase in THC and either stable or decreasing levels of CBD, potentially making cannabis more harmful.

"These changes in the illicit market are largely hidden from scientific investigation and are difficult to target by policy-makers.

"An alternative option could be to attempt to control THC and CBD content through regulation."

Around 24 million people in Europe - 7.2% of the continent's adult population - are estimated to have used the drug in the last year.

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

The research into European cannabis is published in the journal Addiction.