At 3 months old, Marissa Parsons began having frequent seizures, some lasting a half-hour. For almost two decades, her desperate parents tried drug after drug to treat her rare type of childhood epilepsy, but the medications mostly produced side effects — pancreatitis, hives and extreme drowsiness — and she was frequently hospitalized. Family outings became almost impossible. Eventually, her mother said, Marissa stopped smiling.

The recreational use of marijuana became legal in California on Jan. 1. The state is now home to the country’s largest legal pot market.