Opioid painkillers such as morphine, codeine, methadone and oxycodone are a complicated family of drugs that pose a serious addiction risk for many people. The state's largest insurer, Blue Cross Blue Shield of Massachusetts is aiming to curb those risks with new regulations that tighten control on access to these drugs. The rules, which took effect Sunday, encourage patients to seek alternatives before asking for opioids, and, in theory, make it harder for doctors to overprescribe the drugs. The rules aim to reduce addiction, but doctors are concerned that the blanket changes may create another risk: under-medication.