Age of onset for cannabis use and brain development seems to be key, said lead researcher Madeline Meier, a post-doctoral researcher at Duke University in Durham, N.C.
The researchers looked at how IQ scores changed between ages 13 and 38 in more than 1,000 participants from New Zealand who were born in 1972 and 1973 and followed until age 38.
The key variable was the age of onset for marijuana use and the brain's development, Meier said.
Before the age of 18, the brain is still being organized and is thought to be be more vulnerable to damage from drugs.
Persistent cannabis use appeared to affect everyday cognitive functioning, with persistent users showing more attention and memory problems than other participants, the researchers reported in Monday's online issue of the PNAS.
About five per cent of the study group were considered marijuana-dependent, or were using more than once a week before age 18. A dependent user is one who keeps using despite significant health, social or family problems.
The participants had tests including memory, reasoning and processing speed.
IQ scores dropped only in those who became dependent by age 18, the researchers found, after taking years of education and use of other drugs including alcohol into account.
The scores dropped by an average of eight points among those considered dependent in three or more surveys.
For someone of average intelligence, an eight-point drop would mean ranking higher than 29 per cent of the population instead of 50 per cent, Meier said.
Earlier studies did not measure brain function before marijuana use began.
"I think this is the cleanest study I've ever read" that looks for long-term harm from marijuana use, said Dr. Nora Volkow, director of the U.S. National Institute on Drug Abuse, which helped fund the research along with the British government, and a foundation in Zurich.
Ken Winters, a psychiatry professor at the University of Minnesota and senior scientist at the Treatment Research Institute in Philadelphia, said the new findings aren't definitive, but they underscore the importance of studying how marijuana may harm young people. He had no role in the work.