Congressional efforts to force the companies to hand over samples of their drugs to generic competitors have not been successful. Recent proposals to ensure generic access to drug samples would save the federal government $3.8 billion over 10 years, according to the Congressional Budget Office, partly by lowering Medicare and Medicaid spending on prescription drugs.

The Federal Trade Commission has also been investigating the practice.

Some brand-name drug makers say the legislation is not needed. The F.D.A. has been speeding up approval of generic drugs, and nearly 90 percent of prescriptions are filled with generic drugs.

But others point to the ways in which major pharmaceutical companies have managed to keep patents on certain expensive drugs for years longer than necessary.

Generic drug developers usually need between 1,500 to 5,000 units of the brand drug to develop their product and test it, to show that it is effective and can be absorbed at the same rate as the brand drug, according to the agency. Both the F.D.A. and the Federal Trade Commission say securing the samples can be difficult, because major drug firms can invoke safety concerns — real or unreal — to avoid providing the materials.