The Food and Drug Administration has called for the toughest safety warning on two diabetes drugs, Avandia and Actos, whose health risks have become a docus of Congressional concern.

That decision, disclosed on Wednesday by the FDA commissioner at a packed House hearing, comes more than a year after the agency's safety reviewers strongly recommended such a step. And it occurs amid a Congressional investigation into why the agency delayed its warnings about Avandia for years.

When a Congressional committee holds a hearing next Wednesday, the subject will be the safety of the diabetes drug Avandia and whether federal drug regulators have paid close enough attention to its potential risks.

But for one witness who is scheduled to appear, Dr. John B. Buse, a nationally noted diabetes specialist, the hearing will take a different turn, focusing on whether he was the target of an effort by the drug's maker, GlaxoSmithKline, to silence his criticism of the drug.

Federal regulators did not act on a recommendation from their safety reviewers several months ago that GlaxoSmithKline's diabetes drug Avandia should carry the strongest possible caution about heart attacks, Senator Charles Grassley said Thursday in a statement.

Mr. Grassley, Republican of Iowa, said in a statement that some Food and Drug Administration staff members had concluded that prescribing information for the medicine should include a warning framed by a black box, the most serious type of alert.