Teenagers who frequently use cannabis may experience a decline in their IQ over time, new Irish research has found.

According to the findings, those who used cannabis frequently during adolescence experienced declines of approximately two IQ points over time, compared to those who did not use the drug.

Researchers at the Royal College of Surgeons in Ireland (RCSI) and Beaumont Hospital in Dublin carried out a review and analysis of seven studies involving over 800 young people who had used cannabis at least weekly for a minimum of six months. They were compared to more than 5,300 young people who did not use the drug.

The young people were followed up until they were an average of 18 years old, although one study followed its participants until they were 38 years old.

The study found that as well as declines of around two IQ points over time in those who had used cannabis regularly during adolescence, this decline appeared to be mainly related to a reduction in verbal IQ.

"Previous research tells us that young people who use cannabis frequently have worse outcomes in life than their peers and are at increased risk for serious mental illnesses like schizophrenia.

"Loss of IQ points early in life could have significant effects on performance in school and college and later employment prospects," commented the study's senior author, Prof Mary Cannon, of the RCSI.

According to the study's first author, Dr Emmet Power of the RCSI, the use of cannabis by young people is a cause of major concern because "the developing brain may be particularly susceptible to harm during this period".

"The findings of this study help us to further understand this important public health issue," Dr Power said.

Details of these findings are published in the journal, Psychological Medicine.