Middle-aged adults whose memories have grown hazy can't blame occasional pot smoking or other light illicit drug use for their forgetfulness, according to a British study, although experts warn heavy, prolonged use could harm mental functions.

The study, carried in the American Journal of Epidemiology, tested the mental function and memory of nearly 9,000 Britons at age 50 and found that those who had used illegal drugs as recently as in their 40s did just as well, or slightly better, on the tests than peers who had never used drugs.


Marijuana was by far the most common indulgence for the participants—who were surveyed at age 42 about current or past drug use, then tested at age 50—with six percent saying they had used it in the past year, while one-quarter said they had ever used it.

Other drugs they were asked about included amphetamines, LSD, hallucinogenic mushrooms, cocaine and ecstasy—with anywhere from three percent to eight percent of study participants saying they'd ever used those drugs.

"Overall, at the population level, the results seem to suggest that past or even current illicit drug use is not necessarily associated with impaired cognitive functioning in early middle age," said lead researcher Alex Dregan, of King's College London.

"However, our results do not exclude possible harmful effects in some individuals who may be heavily exposed to drugs over longer periods of time."

A small subset of participants who said they had ever been treated for their drug use, which could suggest heavy or addicted drug use, did not fare as well cognitively at 50, but there were too few of them to draw meaningful conclusions, the study authors noted.

Dregan's team used data on 8,992 42-year-olds participating in a UK national health study, who were asked if they had ever used any of 12 illegal drugs. Then, at the age of 50, they took standard tests of memory, attention and other cognitive abilities.

Overall, the study found, there was no evidence that current or past drug users had poorer mental performance. In fact, when current and past users were lumped together, their test scores tended to be higher.

But that advantage was small, the researchers said, and might just reflect another finding—that people who'd ever used drugs generally had a higher education level than non-users.

"In a Western population of occasional drug users, this is what you'd expect to see," said John Halpern, a psychiatrist at Harvard Medical School and McLean Hospital in Belmont, Massachusetts, who has studied the potential cognitive effects of drug use.

"In some ways, this is not surprising. The brain is resilient."

Though some studies have found that drugs like marijuana and cocaine may cloud thinking, memory and attention in the short term, the current findings support the notion that those effects may be temporary, Dregan's team said.

Halpern noted that work focusing on people who have smoked pot regularly for years showed that once they stop the drugs, their deficits on cognitive tests improve after a month.

Still, he said this should not be taken as an endorsement of drug use, noting that the current study did not rule out the possibility of lasting negative cognitive effects from heavy, prolonged drug use.