The rise of herbal cannabis is at the expense of cannabis resin, which traditionally has been the main form of cannabis consumed in Europe and mostly comes from Morocco.
The European Monitoring Centre for Drugs and Drug Addiction report said this shift is against the general background of a stabilisation in cannabis consumption in Europe, with some countries even reporting a drop in use.
The report said some of the explanation could be the reduction in tobacco smoking, partly due to smoking bans in many EU countries, including Ireland.
EMCDDA director Wolfgang Goetz said the shift to domestically produced herbal cannabis “appears to have happened quickly”.
He said: “We know that home-produced cannabis is often more potent, and that controlling home production is a more difficult challenge for law-enforcement services.”
EMCDDA analyst Paul Griffiths said the production facilities were harder to uncover and were located within communities.
“It’s harder for police to detect and interdict, as no trans-border trafficking is involved and production sites are located close to the consumer and are relatively easy to conceal.”
There have been a number of operations in Ireland this year, where cannabis production factories have been uncovered along the border.
The facilities were in remote locations and had been kitted out with sophisticated and expensive equipment used in the heating and watering of cannabis plants.
Most of those arrested were from Asia, typically Chinese and Vietnamese, following a similar trend in Britain in recent years.
A recent British survey estimated 80% of cannabis being used there was home-grown herbal cannabis, compared with 20% in 2000.
Mt Goetz said he was particularly concerned at the estimated four million Europeans who were taking cannabis daily.
The EMCDDA report said the possible links between the drop in cannabis consumption and the reduction in tobacco use needed further examination.