A new pain reliever that uses marijuana extracts can be prescribed in Britain, the government said this week. Under the new policy, the drug Sativex can be given to multiple sclerosis patients to relieve pain and spasms. For now, British patients must import their Sativex from Canada. But GW Pharmaceuticals, the company that makes Sativex, has been given a license to cultivate pot in the U.K. and plans to open a factory there. The plants are already being grown at a secret location somewhere in England.