Certainly, and I don't think anyone would advocate cannabis use for young people. The study that you referred to, the Dunedin study, is one of the studies where substantial concerns have been raised about confounds. With the criminalization of cannabis, we see people who are more likely to violate all kinds of rules using cannabis. So it's not the effects of cannabis per se, but rather confounds related to socio-economic status or other personality factors related to rule-breaking and anti-sociality that may account for some of those IQ changes. So the Dunedin study, I think, is problematic and there have been publications to that effect.
On May 13th, 2014. See this statement in context.