Doctors say they are thinking twice about prescribing an Abbott Laboratories drug used to raise levels of good cholesterol to patients taking a statin pill that is successfully lowering their bad cholesterol. The change of heart by physicians about the need for Abbott's Niaspan follows the release last week of a National Institutes of Health study that showed the drug failed to prevent heart attacks and slightly raised the risk of a stroke when combined with the popular generic cholesterol pill simvastatin, also known by the brand name Zocor. Doctors say statins, including simvastatin, Lipitor and Crestor, do such a good job at lowering LDL, or bad cholesterol, that there's often little need to add another pill to most patients' treatment. And the study is giving them more reason not to add Niaspan, which raises HDL, or the good stuff.