Herbal cannabis produced in Ireland has a higher potency than imported varieties, according to a study by the Forensic Science Laboratory (FSL) on behalf of the National Advisory Committee on Drugs (NACD).

The study analysed cannabis products to establish their tetrahydrocannabinol (THC) content. THC is the main psychoactive component in cannabis. Cannabis is the most commonly used illicit drug in Ireland.

Because of Ireland’s relatively cold climate the plants cannot be grown outdoors here and so they are grown indoors using hydroponics and intensive lighting equipment. This growing method produces higher THC concentrations.

In many cases, female rather than male plants are used as they cannot produce seeds, meaning all the energy within the plant goes into the production of THC, increasing the plant’s potency.

“Many of the plants being grown here are genetically selected to ensure they produce high levels of THC but they also lack a substance called cannabidiol (CBD), which seems to protect the brain from the effects of THC. The effects of the latter include psychosis,” according to NACD chairman Dr Des Corrigan.

While the majority of cannabis products seized by gardaí are thought to be imported from countries where the plant is grown for exportation, the NACD is concerned about the rise in cannabis cultivation here.

Discoveries of large so-called “cannabis factories” have increased in recent years with seizures of approximately 20,000 plants in recent years.

“Samples from seizures of the cannabis herb in Limerick, Cork, Tipperary, Bandon, Fermoy and Dublin – including Ronanstown, Dundrum, Tallaght and Crumlin – found Irish cultivated cannabis had very high THC levels and very low CBD levels compared to imported herb and resin."

High THC levels in cannabis raise serious health concerns, according to Dr Corrigan.

“Recent studies in the UK have shown that there is a higher risk of psychosis in those who smoke high-potency cannabis products compared to those who smoke hash which contains both THC and CBD,” he said.

“Monitoring the potency of cannabis is extremely important. The NACD hopes to continue its collaboration with the FSL so as to present an annual monitoring report on cannabis potency which will track an emerging public health issue and which will allow for comparison of future results,” he concluded.