The increase coincided with a general rise in the potency of the popular recreational drug and a growing belief that it is not harmful, researchers wrote in The Lancet Psychiatry.
The findings, the US-based team wrote, “suggest a potential benefit of education and prevention messages” even as many US states are relaxing cannabis policies.
Based on a survey of over 500,000 US adults between 2002 and 2014, the study found that marijuana use rose from 10.4 percent of the population in 2002 to 13.3 percent in 2014 — from 21.9 million to 31.9 million.
The number of daily or near-daily users was about 8.4 million in 2014, they estimated — up from 3.9 million in 2002.
The proportion of people who said they feared great risk or harm from smoking dope once or twice a week, dropped from 50.4 percent to 33.3 percent over the same period.
The study did not find a rise in so-called marijuana use disorders such as abuse or dependence. The study did not look at cannabis use among children or teenagers.
Critics of decriminalisation have argued it will cause more people to take up the drug, which is partly what prompted the study.
“These changes in the prevalence of cannabis use occurred during a period when many US states legalised cannabis for medicinal use, but before four states went on to legalise recreational cannabis use,” addiction experts Michael Lynskey and Wayne Hall wrote in a comment also carried by the journal.
“It is probably too soon to draw conclusions about the effects of these legal changes on rates of cannabis use and cannabis-related harms, but it is likely that these policy changes will increase the prevalence and frequency of cannabis use,” they said.
The trend is not a global one — cannabis use in Britain has gone down in the last 10 years, according to Robin Murray of Kings College London, who also commented on the study.