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More parents are turning to cannabidiol (CBD) oil, a cannabis extract with little or none of the psychoactive compound THC, to treat their children who have cancer and epilepsy. The oil is currently legal in dozens of states, but its supply is limited. And the science lags the law—dosing standards haven't been set, and the effects of long-term use are unclear. Follow Penn and Nicole Mattison and their daughter Millie, Tracy and Josh Ryan and their daughter Sophie, and Sarah and David Rowland and their daughter Lily as they navigate the challenges of using CBD oil.