The agency announced it had filed charges against two companies for promoting the shields with unsupported claims such as "prevents electromagnetic waves from penetrating the brain" and "blocks up to 99 percent of the radiation."
"There is no scientific evidence that their products work as they claim," said Howard Beales, the FTC's director of consumer protection. "These companies are using a shield of misrepresentation to block consumers from the facts."
Last year, congressional investigators reported that current research doesn't show the energy emitted by cell phones is harmful, but "there is not yet enough information to conclude that they pose no risk."
The report from the General Accounting Office said long-term studies now under way may offer firmer answers, but it will likely be years before it is known for sure whether cell phones are a health risk.
But even if there is a danger, cell phone shields are not the solution, Beales said.
The FTC said shielding products that block only the earpiece are ineffective because the entire phone emits radiation. Shields may also interfere with a phone's signal, causing it to draw extra power to communicate and emit more energy.
The agency said cell phone users who want to limit exposure to electromagnetic emissions should limit cell phone use, use a handsfree headset and avoid using cell phones where the signal is weak.
The number of cell phone subscribers has surged to more than 131 million, according to the Cellular Telecommunications and Internet Association, the leading industry trade group.
CTIA spokesman Travis Larson said the industry agrees with the government advisory. He said cell phones already meet federal safety guidelines and devices promoted as shields don't make phones safer.
"Companies are creating and exacerbating public fear to create a market for their product," Larson said. He said the shield products are not subject to government review and manufacturers often cite isolated research reports that don't reflect most scientific findings on cell phone safety.
The companies named in the FTC's complaint are Stock Value 1 Inc., of Boca Raton, Fla., and Comstar Communications Inc., of West Sacramento, Calif. In the complaints, filed last week in Florida and California, the government said it wants to shut down the companies and have them give refunds to customers.
The companies could not be reached for comment and their lawyers did not immediately return calls.
Both companies advertised through television, radio, print ads and the Internet, making false claims about scientific proof backing their products, the FTC said.
The Stock Value 1 promoted products called "SafeTShield" and "NoDanger," both metallic fiber patches for placement over cellular and cordless telephone earpieces, the FTC said. Comstar described its "WaveShield 1000" product as about the size of a penny and able to block "radiation entering the soft tissue of the ear canal."
Tests by the Good Housekeeping Institute, which referred the cases to the FTC, found that none of the products reduced radiation exposure, the agency said.