Valerie Curran, professor of psychopharmacology at University College London and a member of the UK's Independent Scientific Committee on Drugs, was more sceptical, saying that a other factors, such as depression, are also associated with heavy use and reduced motivation. “Although the overall sample size is excellent, the data on adolescent onset of heavy use is based on just over 50 people.”
She also noted that the findings represent a very small decline in IQ as a result of very heavy use over a number of years, “which doesn’t relate to recreational use”.
Recent surveys have shown that fewer adolescents believe that regular cannabis use is harmful to health. They are starting to use cannabis at an earlier age and more of them are using it every day.
With this in mind, the authors say that their findings suggest that policy-makers should put more emphasis on attempting to delay the point at which teens start smoking cannabis.
“Let’s keep some perspective. This is not huge," Murray says. "You’re not becoming completely demented. Of course, it’s better to still have your extra eight IQ points, but it’s not something that’s enough to draw medical attention.”
The effects of the significant increase in the amount of the active ingredient, tetrahydrocannabinol (THC), in the drug over the past few decades also needs to be explored. “You have to remember that when the people in the Dunedin study were kids back in the early eighties, this would have been pretty old-fashioned hash or weed, with THC content of 4–5%," Murray points out. "Today’s skunk has 16–18%, so the effects are most likely to be magnified.”