Fears that cannabis causes irreparable harm to teenager’s brains have been stoked by trials which “overstated” the effects on intelligence and other functions, according to a review which found little ill-effect after three days abstinence.

Studies have shown it is 114 times less harmful than alcohol, but marijuana’s impact on adolescents’ brain development and mental health is a major concern for policy makers in debates over legalisation.

This is a key time developmental period and studies have found negative impacts on attention, learning, memory and organisation in heavy or frequent cannabis users.

But it now appears that these studies may have just been identifying impairment caused by residual effects of the drug or withdrawal symptoms.

A meta-analysis by University of Pennsylvania researchers looked at 69 studies, which all tested the impact of marijuana smoking in adolescents and young adults, and found little to no long-term harm.

Although it did not look at mental health impacts, such as psychosis, the authors of the study published in the journal JAMA Psychology today, found the “persistence and magnitude of impact” on teenagers had been overblown.

For the test the authors looked at results of cognitive testing on learning, memory, speed of information processing, language and motor skills for 2,152 cannabis users and 6,575 who had minimal use.

Unexpected successful people who have admitted to smoking cannabis Show all 14 1 /14 Unexpected successful people who have admitted to smoking cannabis Unexpected successful people who have admitted to smoking cannabis Barack Obama "As has been well documented, I smoked pot as a kid, and I view it as a bad habit and a vice, not very different from the cigarettes that I smoked as a young person up through a big chunk of my adult life. I don’t think it is more dangerous than alcohol.” Getty Unexpected successful people who have admitted to smoking cannabis Bill Maher "Look, I have never made a secret of the fact that I have tried marijuana... About 50,000 times." Getty Images Unexpected successful people who have admitted to smoking cannabis Boris Johnson "It was jolly nice. But apparently it is very different these days. Much stronger. I've become very illiberal about it. I don't want my kids to take drugs." AP Unexpected successful people who have admitted to smoking cannabis Bill Clinton "I experimented with marijuana a time or two." Reuters Unexpected successful people who have admitted to smoking cannabis Jon Stewart “Do you know how many movies I wrote when I was high?” Comedy Central Unexpected successful people who have admitted to smoking cannabis Harriet Harman "I did, when I was at university, smoke cannabis once or twice." But she quickly added she had not taken the drug since then. "I have indulged in the odd glass of wine but not cannabis." PA Unexpected successful people who have admitted to smoking cannabis Martha Stewart “Of course I know how to role a joint.” Getty Images Unexpected successful people who have admitted to smoking cannabis Bill Gates In Stephen Manes’ biography about the Microsoft founder, he writes: “As for drugs – well, Gates was certainly not unusual there. Marijuana was the pharmaceutical of choice…” Chip Somodevilla/Getty Images Unexpected successful people who have admitted to smoking cannabis Oprah Winfrey Asked by Andy Cohen when the last time she’d smoked cannabis was, she answered: “Uh … 1982.” “Let's hang out after the show,” Cohen joked. “Okay,” Winfrey said, laughing. “I hear it's gotten better.” Getty Images Unexpected successful people who have admitted to smoking cannabis George W. Bush "I wouldn't answer the marijuana questions. You know why? Because I don't want some little kid doing what I tried." Getty Unexpected successful people who have admitted to smoking cannabis Sarah Palin "I can't claim a Bill Clinton and say that I never inhaled.” AP Unexpected successful people who have admitted to smoking cannabis David Letterman “I went through one period when I smoked a surprising, a really breath-taking, amount of grass almost every night.” Getty Images Unexpected successful people who have admitted to smoking cannabis Morgan Freeman "Never give up the ganja." Getty Images Unexpected successful people who have admitted to smoking cannabis Steve Jobs “The best way I would describe the effect of the marijuana and the hashish is that it would make me relaxed and creative.”

It found there was a “small but statistically significant” effect associated with heavy or frequent use.

But this effect shrank in studies that required their cannabis-smoking participants to abstain from smoking before testing, and a period of more than 72-hours abstinence meant there was no significant impact on their performance.

It also found no evidence that earlier use, particularly in teenagers, had a more serious long-term effect.

“In light of the changing perceptions of cannabis use and an evolving policy landscape surrounding cannabis, understanding the potential risks of cannabis use for mental health and brain functioning is of paramount importance,” the authors conclude.

“[Our] results indicate that previous studies of cannabis in youth may have overstated the magnitude and persistence of cognitive deficits associated with use.