Casual Pot Use Causes Brain Abnormalities

This Chicago Tribune article entitled "Casual Pot Use Causes Brain Abnormalities in Young: Study" reports on a recent research study revealing that even with casual use, there is a direct correlation between the number of times young users smoked and brain abnormalities, especially in the nucleus accumbens and the nucleus aygdala. These are two areas which regulate emotion and motivation. Is there any "safe" level of pot smoking? What are the implications, especially if people use medical marijuana?