Cannabis warning as major study finds two thirds of drug users suffer reduced brain activity

Almost two thirds of people who regularly use cannabis are likely to suffer issues with brain function, concerning research today suggested. 

Advocates of the drug often purport that marijuana is safe because it is 'natural'.

But US scientists, who tracked over 1,000 young adults, found 63 per cent of heavy cannabis users had reduced brain activity during memory tasks. 

The figure stood at almost seven in 10 among recent users. 

Experts, who claimed it was the largest study of its kind, urged users to reflect on the risk of consistent cannabis use but warned further, longer-running research was still vital. 

In the study, 88 participants were classed as heavy cannabis users, with 179 moderate users who were then compared to 736 non users. 

Heavy users were considered those who had used cannabis more than 1,000 times over their lifetime.  

Researchers studied their brain activity via MRI scans as all participants undertook seven different tasks. 

These tested working memory, emotional response, language, motor skills – with activities including tasks such as tapping a finger to map brain control.

The researchers found that cannabis had a statistically significant effect on brain function during working memory tasks, with 63 per cent of heavy users showing reduced brain activity. 

This, the scientists said, was 'very unlikely to be due to random chance'. 

During working memory tasks, the researchers also said that heavy users appeared to have reduce brain activity in specific areas of the brain — the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex and anterior insula. 

These regions are involved in important cognitive functions such as decision-making, memory, attention and emotional processing. 

A similar effect on working memory was seen in moderate cannabis users. But the impact was less significant across the other tasks.  

Writing in the journal JAMA Network Open, the researchers said: 'Our findings highlight the need to educate cannabis users about the consequences of recent and heavy lifetime cannabis use on cognitively demanding working memory tasks. 

'Similarly, the association between heavy use and decreased brain function could motivate regular cannabis users to reduce their cannabis use and could encourage treatment.'