A 2012 study linking persistent cannabis use to neuropsychological decline has been discredited. 

In a study published in the journal PNAS in 2012, scientists from Duke University in the US reported that cannabis had a neurotoxic effect on the adolescent brain, causing a loss of up to 8 IQ points in the heaviest users. They found that IQ, learning, memory, and executive functions all declined in heavy users who started smoking before the age of 18.

The research was part of a longitudinal study of 1,037 New Zealand children born between 1972 and 1973. Health, intelligence and behaviour measures were taken periodically on all participants, most recently at the age of 38. 

The scientists warned that there may be other explanations for their findings and that they could not "definitively attest to whether this association [between persistent cannabis use and IQ decline] was causal". The study did rule out factors such as years of education, schizophrenia, hard-drug and alcohol dependence, but did not account for other relevant factors such as childhood trauma.

A study published in PNAS six months after the original research questioned the methodology, saying: "Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature."

The researchers argued that if you take into account the effects of socioeconomic status on IQ, the decline in IQ due to cannabis use is overestimated by the previous study - and could in fact be zero. 

A new study by the University College London has now shed further light on the Duke University findings. This study drew on a larger sample of adolescents; 2,612 UK children born between 1991 and 1992. They found that heavy cannabis use was in no way linked to IQ decline, although alcohol use was strongly correlated with IQ loss in eight to 15 year olds. 

There was some evidence that cannabis use impacted school test scores: the heaviest users scored 3 per cent lower on exams at age 16. This mirrors other research, which has linked heavy marijuana use to poor academic performance and low school completion rates in teenagers. 

However, a new study in California where cannabis was recently legalised show that legalising the drug leads to no increase in problematic adolescent behaviour such as crime, drug overdose, driving under the influence, or school dropout rates. This indicates that even if negative health effects are a consequence of smoking cannabis, prohibition might not be the best option. 

Sources: The Washington Post