"It needs to be emphasised that regular cannabis use, which we consider once a week, is not safe and may result in addiction and neurocognitive damage, especially in youth," said Krista Lisdahl, director of the brain imaging and neuropsychology lab at University of Wisconsin-Milwaukee.
People who have become addicted to marijuana can lose an average of six IQ points by adulthood, according to Lisdahl, referring to a 2012 longitudinal study of 1,037 participants who were followed from birth to age 38.
Abnormalities in the brain's gray matter, which is associated with intelligence, have been found in 16- to 19-year-olds who increased their marijuana use in the past year, she said.
These findings remained even after researchers controlled for major medical conditions, prenatal drug exposure, developmental delays and learning disabilities, she added.
Tetrahydrocannabinol, or THC, is responsible for most of marijuana's psychological effects, researchers said.
Some research has shown that frequent use of high potency THC can increase risk of acute and future problems with depression, anxiety and psychosis.
"Unfortunately, much of what we know from earlier research is based on smoking marijuana with much lower doses of THC than are commonly used today," Budney said.
The research was presented at the American Psychological Association's Annual Convention.
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