A study claims that young cannabis smokers run risk of lower IQ. The findings come from a study of around 1,000 people in New Zealand. For more than 20 years researchers have followed the lives of a group of people from Dunedin in New Zealand. An international team found those who started using cannabis in adolescence suffered a drop in IQ. This might might have affected their still developing brains. This study also showed that cognitive performance was restored when the use of cannabis was stopped.