>> reporter: kerry kennedy, daughter of robert kennedy, hit a tractor trailer in july of 2012 after mistakenly taking an ambien. >> i want to apologize to the driver of the truck who i apparently hit. >> reporter: she was found not guilty of impaired driving. the study also finds the risk of accidents increases over time, and can last up to a year after you start taking the drugs. >> it's not just a risk to them if they are out driving, but it's also to each and every one of us out on the road. >> reporter: this problem led the fda two years ago to recommend women take lower doses of ambien because they metabolize the drug slower than men. the makers of ambien and restoril say their medications are safe and effective when used according to the label, making sure those on the road are aware of lingering risks from the night before. anne thompson, nbc news, new york.