The potency of cannabis consumed across Europe has doubled in the past decade, according to a new study from academics in the UK.

Herbal cannabis potency rose from 5% in 2006 to 10% in 2016; cannabis resin potency increased from 8% in 2006 to 17% a decade later.

They collected data from all 28 EU member states, plus Norway and Turkey, and looked into the concentration of THC. For herbal cannabis it rose from an average of 5% in 2006 to 10% in 2016, while for cannabis resin it increased from 8% in 2006 to 17% a decade later.

The academics, working for the European Monitoring Centre for Drugs and Drug Addiction, found the price has risen, but not in line with the increase in potency. Herbal cannabis went from €7.36 per gram in 2006 to €12.22 in 2016, while resin rose from €8.21 per gram to €12.27.

The findings were published in the journal Addiction and lead author Tom Freeman, from the University of Bath, said: “These findings show that cannabis resin has changed rapidly across Europe, resulting in a more potent and better value product.”

Yet he also argued that cannabis sold on the black market is becoming more harmful, as CBD is not rising in line with THC. CBD is the compound that brings a number of health benefits and offsets side effects like paranoia that can be associated with the psychoactive THC.

Freeman noted that CBD can make cannabis safer, without limiting the positive effects that its users pursue. New techniques in Morocco and Europe have allowed growers to increase THC, but not CBD, and it is difficult for lawmakers to affect change. Freeman said that governments could attempt to control THC and CBD content through regulation, which would amount to legalizing and controlling cannabis in a number of markets.

The group estimated that 7.2% of European adults use cannabis, highlighting a vast potential market for a legal cannabis industry.

A separate study in the Addiction journal found that legalizing cannabis is highly unlikely to spark a rise in the black market. It concluded that cannabis price policies that include somewhat higher consumer costs for legal cannabis relative to contraband would not be expected to incentivize and expand the illegal market.