In an effort to curb abuse of powerful painkillers, Massachusetts's largest insurer is going to restrict doctors' ability to write new prescriptions to 30 days' worth of pills before a mandatory review by the insurer. Officials at Blue Cross Blue Shield of Massachusetts say the policy, which begins July 1, will prevent doctors from prescribing 60 days of a painkiller such as OxyContin, for example, for a minor problem such as a sprained ankle. Patients with such serious or chronic conditions as cancer or those who are terminally ill will be permitted to continue to receive opioid painkillers.