Marijuana use over time is associated with remembering fewer words but doesn't appear to harm other cognitive functions, new research shows.

"Whatchamacallit?" may be uttered more frequently by long-term marijuana users than by those who don't smoke the drug habitually.

New research findings suggest that regular marijuana use over time is associated with poorer word memory in middle age but doesn't appear to affect other areas of cognitive function.

The US prospective study, published in JAMA Internal medicine, related to 3385 middle-aged adults who were followed up for 25 years.

While 2852 reported past marijuana use, only 392 continued to use marijuana into middle age.

For every five years of past exposure, lower verbal memory corresponded to an average of one or two participants remembering one word fewer from a list of 15 words.

The researchers took into account other factors that may have affected cognitive performance.

In an associated commentary, the University of Queensland's Professor Wayne Hall said the findings fitted well with other evidence on the effects of long-term marijuana use on cognitive performance.

"Case-control studies have generally found poorer verbal learning, memory, and attention in those who regularly use marijuana than in controls," he said.

"The public health challenge is to find effective ways to inform young people who use, or are considering using, marijuana about the cognitive and other risks of long-term daily use.

"Young adults may be sceptical about advice on the putative adverse health effects of marijuana, which they may see as being overstated to justify the prohibition on its use.

"More research on how young people interpret evidence of harm from marijuana and other drugs would be useful in designing more effective health advice."