A U.S. federal appeals court said Friday it could not force the Food and Drug Administration to tighten restrictions on dental fillings containing mercury.

Advocacy groups sought to ban the use of such fillings and to force the FDA to classify them as risky, subjecting them to tougher regulations.

The groups say the fillings pose health risks to patients who inhale mercury vapors and dental office employees who handle the materials.

The U.S. Court of Appeals for the District of Columbia Circuit unanimously ruled that, while certain FDA actions can be appealed, the court has no jurisdiction to review agency inaction.

The mercury mixture has stirred controversy since dentists began using it to fill cavities in the 1800s. Significant levels of mercury exposure can cause permanent damage to the brain and kidneys, but the FDA has said for years that mercury fillings don't harm patients, except in rare cases when they have allergic reactions.

Amalgam fillings are about 50 percent mercury, joined with silver, copper and tin. Tens of millions of Americans receive mercury fillings each year. Many doctors have begun switching to resin composite fillings that blend better with the natural coloring of teeth.

Federal health officials began a new review of the safety of the fillings last year.