dopamine nart of our brain that we call the reward center. >> reporter: in the case of 23- year-old marc bullard, his death certificate lists a contributing factor to "use of concentrated marijuana products". and the autopsy report showed high levels of active t.h.c. in his body. but even marc's parent's realize there is no way to know if it was thc that caused his slide into despair. >> low doses of t.h.c., we know what happens. these super high concentrations of t.h.c., we don't know what happens because we have not been studying it. and when they smoke that they're getting in the 600 to 800 milligrams of t.h.c. >> reporter: that's compared to a limit of 10 milligrams in each serving of an edible in colorado, or maybe 25 milligrams of t.h.c. in a typical marijuana cigarette. but it isn't just mental health that is of concern. brandon cullip was 17, had only had his driving permit for a week, and his friends in the car told him he was too high to drive, but that didn't stop him. cullip pleaded guilty to vehicular homicide, and reckless driving in the death of 16-year old chad britton. cullip is