As the federal government advocates increased use of generic drugs, concerns are mounting about the U.S. Food and Drug Administration’s oversight and the quality or effectiveness of some generics. In the last eight months, the FDA has acknowledged that two generic versions of the attention-deficit/hyperactivity disorder drug Concerta it approved may not work as effectively as the brand-name product. The agency told the drugs’ manufacturers to confirm their effectiveness or withdraw them from the market. The FDA also is looking into findings by a researcher at Boston’s Brigham and Women’s Hospital that generic versions of Pfizer’s cholesterol-lowering drug Lipitor manufactured outside of the U.S. contains impurities that inhibit the drug’s therapeutic effect. The FDA said that, as a number of popular drugs come off patent through 2015, it lacks the resources to independently police generics.