Smoking Weed Regularly Lowers IQ in Teens
By on August 27th, 2012

Cannabis or marijuana, or ‘weed’, as it is known colloquially, is constantly in the news both for its toxic effects and conversely because it has also been found to have certain medicinal effects. While its use as a recreational drug is generally outlawed, there are lobbies to make its use legal in certain parts of the world. Recent experiments are revealing that the use of cannabis at a young age can lead to significant and enduring cognitive impairment.

What Has Been Studied Previously

There have been previous studies testing the effects of regular cannabis use on the functions of the brain. However, one limitation of most studies has been that they only looked for differences between two groups of people—one which consumed cannabis, and another which didn’t. One problem with such studies is that the difference between the two groups might be due to a factor other than cannabis consumption. For example, if you were testing IQ levels of cannabis users versus non-users and found that there was a difference between the two groups, this might be because a majority of people who use cannabis do not attend school and thus have lower IQs (it has been previously shown that education increases IQ).

Cannabis smoking could lead to a significant decline in IQ in teens. [Image Credit: Wikipedia]

In this study, a team of researchers from the United Kingdom and New Zealand had access to neuropsychological data from around 1000 New Zealanders from 1985-1986 (when none of them used cannabis). The same people underwent neuropsychological tests in 2012, by when some of them were cannabis users. In their analysis, the researchers ensured that education levels, tobacco smoking, alcohol consumption and the use of other drugs did not confound the results.

Reduction in IQ by 8 Points

They saw that persistent use of cannabis over 20 years resulted in a significant cognitive decline, which was more severe when its use started in adolescence. The most persistent cannabis users saw an average 8 point decline in IQ. More attention and memory problems which affected daily life were observed in cannabis users. Among those who started the use of cannabis in adolescence, the impairment seen continuing for a year after the consumption of cannabis was stopped.

The brain undergoes critical neural development in adolescence, and cannabis may contain neurotoxins that impede this development. Though the mechanism linking cannabis use to lowering of IQ is not known yet, these results throw further light upon the debilitating effects of this drug. You can read about this research here.

Author: Shweta Ramdas
Beginning life as a grad student studying human genetics.

Shweta Ramdas has written and can be contacted at

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