Suzana Herculano-Houzel

Column_167 Weed does harm the brain

Magazines keep rehashing articles about marijuana, always with a defensive tone, ignoring the ever-increasing evidence: “marijuana is not bad for you”. The worst thing is that more and more young people believe this.

But I’m hopeful that this time things will change. Firstly, because a study of Dutch users showed that heavy marijuana consumption is associated with a reduction in the size of the hippocampus. The higher the consumption, the smaller the hippocampus – which, given the known toxic effect of marijuana’s active ingredient on the neurons in this structure, which are fundamental for the formation of new memories, leads us to believe that these users are killing their neurons.

And secondly: a beautiful study carried out in New Zealand, led by the American Terrie Moffitt, has just shown that the continued use of marijuana starting in adolescence is in fact associated with a reduction in memory, mathematical ability, language, IQ – in short, all the abilities tested.

The study, which exploited the rich database collected over 38 continuous years on all 1037 inhabitants of the city of Dunedin born between 1972 and 1973, compared the cognitive abilities of the participants at various ages, from before any marijuana use to the present. The non-users did not have any significant loss of IQ or cognitive abilities, nor did those who started using marijuana only after the age of 18. But participants who had used marijuana since before the age of 18 suffered a reduction of more than 10% in IQ, and even worse memory losses. Worse still, those who stopped smoking marijuana as adults did not reverse the loss. The damage was already done in adolescence.

I think these studies, added to the already significant body of previous studies, make it clear: marijuana is bad for you. This is a fundamental finding in modern times that favour the legalization of this drug, and Moffitt and his colleagues make a clear call for campaigns to spread the word about it, and to encourage use “only later”.

I’d like to take this opportunity to point out that I’m against the simple decriminalization of marijuana, but I’m in favour of legalizing it, and indeed all other addictive drugs. I find it profoundly stupid to rack one’s brains in exchange for a high – but that should be each person’s personal decision, and without funding the violence that causes problems for others.

Originally published in December 2012 in Folha de São Paulo

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