Living somewhere between the worlds of science and politics has been the longstanding debate regarding the negative effects of cannabis abuse. While there have been some studies linking its use to lower IQ, most have not assessed IQ prior to the onset of substance use.
A recent study published in PNAS will likely spark more debate. The data come from the well-known Dunedin study from New Zealand (you remember that famous gene by environment interaction study in depression, right?) that has followed approximately 1000 individuals from birth to adulthood. IQ was assessed prior to the onset of cannabis use and again at age 38. Cannabis use was ascertained by self-report during an interview.
Results showed that heavy cannabis use was associated with reduced IQ. Specifically, individuals who were regular cannabis users at multiple time points beginning prior to age 18 had lost about 8 IQ points on their score while those who never used has nearly identical scores. The effect was present after controlling for education. Stopping frequent cannabis use later in life did not fully restore this effect.
The authors highlighted the adolescent onset of the persistent cannabis use to argue that this period is particularly vulnerable to any neurotoxic effects of substances such as cannabis and urged increased efforts to delay the onset of cannabis use.
While eight points on an IQ test may not sound like much to some, this effect size is impressive and reflects a sizable difference. At the same time, we can expect that the lack of association between IQ and less intense cannabis use or with adult-onset use will be used by some to argue that moderate amounts of cannabis do not affect intelligence. I guess there is always a way to get data to say what you want it to.
Meier MH et al. Persistent cannabis users show neuropsychological decline from childhood to midlife. PNAS early edition August 2012: 1-8.