Surveys show a small but growing number of older adults are using marijuana - a trend that worries researchers who say not enough information exists about how pot affects older users.
Abundant research has been done on how the drug impacts developing brains, but little is known about the potential consequences on older users.
The researchers reviewed data from the National Survey on Drug Use and Health and found a big increase in adults over 50 reporting they had used pot in the past year.
About 2.8 per cent of those surveyed said they had used pot a decade ago. Six years later in 2013, the rate was about 4.8 per cent - a 71 per cent jump.
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"Historically older people haven't had high rates of substance use, but this is changing," said Dr Benjamin Han, a geriatrician at NYU who led the study published this week in the journal Addiction. "As baby boomers age, we're going to see more and more of this."
Older adults are still much less likely than younger people to use pot. In the 2013 survey, about 19 per cent of people 18 to 25 reported using marijuana in the previous month.
Doctors have little to go on when treating older people who use pot, Han said.
"When it comes to, for instance, alcohol, there have been a lot of studies about effects on older populations, guidelines on how much older people should be consuming," Han said. "But when it comes to marijuana, we have nothing."
The study drew no conclusions about whether older pot users are using the drug as medicine or for fun.
Marijuana legalisation debates centre on the drug's consequences for kids, perhaps leaving adults to think there's no downside to using it.
"Before the liberalisation of marijuana policy, lots of young people used marijuana and then as they got jobs and kids and mortgages, they stopped," said Jonathan Caulkins, a professor of public policy at Carnegie-Mellon University who was not involved in the NYU study.
"It seems that as the social stigmatisation has decreased, more users are continuing into adulthood."
