Actual cannabis data

This blog has a tendency to look pro-drug and pro-alcohol, because we react to media stories and these topics are currently popular for exaggeration and over-interpretation.  It’s a nice change to find some serious anti-drug research that the media are reporting pretty sensibly. (Otago Daily Times, Stuff, Herald, 3News, TVNZ)

Researchers in Dunedin, and their international collaborators, used data from the Dunedin Study, which recruited about 1000 babies born in a Dunedin hospital in the 12 months starting April 1972.  These babies, now adults, have given a lot of data up to medical science. In particular, they had a range of neuropsychological tests at age 13 and then again at age 38, and they were interviewed about cannabis use.   The headline results were about IQ, but the study looked at a relatively broad range of cognitive function tests, and also at self-reported problems with memory and attention.   Participants who smoked cannabis as teens had broadly worse cognitive function, by a margin that’s large enough to be a concern.  This didn’t show up in people who started smoking cannabis as adults.  The paper is in PNAS; the abstract is free.

What’s most distinctive about this study is the before-after comparisons.  You often see (and we have pointed out) studies that claim to have found “changes” when they only had one time point, and so really only looked at “differences”.  The Dunedin Study has changes from before cannabis use to after cannabis use.   Also, since the participants are the same age and all from Dunedin, they are less heterogeneous in many ways than participants in other research studies.

The differences in IQ are within the range of the Flynn effect, so it’s still possible that this is a social effect rather than a biochemical one, or that it’s caused by pre-existing differences in people’s interest in the sort of activities that increase IQ test scores, or something.  Better data, however, will be extremely difficult to collect, and treating these results as probably true seems sensible.  Presumably the adverse effects would also be found with synthetic cannabimimetics, but these are too recent for there to be data, and they have other problems not present with cannabis, such as the risk of acute overdose.

A number of the stories quote Ross Bell, of the Drug Foundation, being sensible:

Simply banning drugs such as cannabis and then thinking that was the problem solved would not work, Mr Bell said. “What we’re lacking in New Zealand is support for widespread, high-quality, well-constructed prevention messages targeted at younger people.”