For decades, cannabis has often been seen as one of the safer recreational drugs. It’s apparently impossible to overdose on, significantly less addictive than other common vices, and despite a 1936 nursing journal warning that anybody intoxicated with the stuff would “suddenly turn with murderous violence upon whomever is nearest … [and] run amuck with knife, axe, gun, or anything else,” studies have found that cannabis users actually have a lower risk of domestic violence than those who abstain.

But does that really mean we can call it a “safe” drug? A new study, based on more than four decades worth of research, suggests not.

“Long-term cannabis users showed IQ decline from childhood to midlife (mean=−5.5 IQ points), poorer learning and processing speed relative to their childhood IQ, and informant-reported memory and attention problems,” reports the paper, published this month in the American Journal of Psychiatry.

“These deficits were specific to long-term cannabis users,” it adds. “[T]hey were either not present or were smaller among long-term tobacco users, long-term alcohol users, midlife recreational cannabis users, and cannabis quitters.”

The data comes from a cohort study that started all the way back in 1972, in Dunedin, New Zealand. Originally designed simply to investigate child health and development in the first three years of life, the Dunedin Study eventually grew into an internationally-renowned resource that has so far produced more than 1,300 studies and research papers on long-term health.

Using the Dunedin Study meant that the researchers were able to collect details about the lives of more than 1,000 individuals, all the way from birth to middle age. That information included participants’ cannabis use and dependence, which had been assessed at age 18, 21, 26, 32, 38, and 45, and their IQ, measured at ages 7, 9, and 11, and then again at age 45.

The results were stark: long-term users performed significantly worse on most cognitive tests than their peers with no history of cannabis use – a discovery which “could not be explained by persistent tobacco, alcohol, or other illicit drug use, childhood socioeconomic status, low childhood self-control, or family history of substance dependence,” the authors note.

Even compared to long-term alcohol or tobacco users and recreational cannabis users, the long-term cognitive effects of cannabis were severe: long-term users showed poorer learning and information processing abilities, as well as reporting significantly more memory and attention problems compared to other groups.

“This study suggests that the cognitive findings are effects of long-term cannabis use, given the consistent results across multiple comparisons and the dose-response relation,” remarked Dr Peter Roy-Byrne, a University of Washington Professor of Psychiatry, who was not involved in the study.

“In my psychiatric practice, I am much less concerned about casual sporadic cannabis use and more worried about regular use,” added Roy-Byrne, who acts as Senior Consulting Editor for the NEJM Journal Watch Psychiatry Board. “[It] appears to aggravate preexisting mood and anxiety disorders in some patients.”

But that isn’t the end of the bad news for long-term cannabis users: the latest Dunedin Study phase included, for the first time, detailed MRI scans of participants’ brains.

“We had been … planning for this project for many years,” the Dunedin research group explains. “Our approach is to use an entire lifetime’s data to look back as far as early childhood and see if there are things that we can know about well in advance that will influence the aging process.”

And when the researchers compared the brain scans of long-term cannabis users to their peers’, they noticed something kind of worrying: the hippocampus – a major brain structure that plays an important role in learning and memory – was significantly smaller in those with the long-term cannabis habits.

Added together, this presents a potentially bleak prognosis for long-term cannabis users. Smaller hippocampal volume and midlife cognitive issues are both known risk factors for dementia – a devastating disease that takes the lives of hundreds of thousands of Americans each year.

“The authors believe that these findings could be clinically consequential, given the link between midlife cognitive decline of similar magnitude and eventual risk for dementia,” noted Roy-Byrne.

“Many patients think of their regular cannabis use as the same as a glass of wine with dinner, but this equivalence appears to be [inaccurate].”