Federal health officials have issued warning letters to eight surgical centers and a marketing firm in California for misleading advertisements promoting the Lap-Band, a stomach-restricting device used to treat obesity.

The Food and Drug Administration said Tuesday that advertisements by the centers and a marketing firm, 1-800-GET-THIN, do not provide mandatory information about the risks and side effects of Lap-Band implantation. The stomach-restricting band _ made by Irvine, Calif.-based Allergan Inc. _ limits food intake, but can also cause irritation of the esophagus, infection, nausea and vomiting in some cases.

FDA said print advertisements and billboards from the companies used large lettering for slogans like "Let your new life begin." However, messages about the requirements and risks of the procedure "appear so small as to render the information illegible."

The companies have 15 business days to respond to the agency and lay out plans to correct the advertisements.

In February the FDA expanded approval for the Lap-Band to an additional 26 million American patients with a body mass index between 30 and 40 and one weight-related medical condition, such as diabetes or high blood pressure. That's up from the roughly 15 million who were eligible under the previous criteria: a body mass index of 40 or higher, or 35 and higher with weight-related complications.

More than a third of all American adults are obese.

The FDA regularly issues warning letters to companies that do not follow regulations for manufacturing and promoting drugs and medical devices. The letters are not legally binding, but the FDA can take companies to court if they are ignored.

1-800-GET-THIN LLC is based in Pasadena, Calif., and provides consultations and information sessions on Lap-Band surgeries.

Calls and email messages to company executives were not immediately returned Tuesday afternoon.