Women with silicone or saline breast implants may face a small increase in risk for a rare immune-system cancer near their implants, U.S. regulators said on Wednesday.

Officials need more data to tell if the implants caused the cancer and are asking doctors to report confirmed cases, the Food and Drug Administration said in a statement. Overall the agency still considers implants safe and said women without symptoms should not change their routine monitoring.

The cancer warning could hit sales of implants sold by Allergan Inc and Johnson & Johnson’s Mentor unit. Allergan shares closed down nearly 1.9 percent.

“Sales growth could be negatively impacted by the media coverage,” Wachovia analyst Larry Biegelsen said in a research note about Allergan, which relies on implants more than diversified healthcare company Johnson & Johnson. Other Allergan analysts said they saw little risk to sales.

Safety concerns have dogged breast implants for years. Silicone implants were banned for most U.S. women in 1992 after some complained the devices leaked and made them chronically ill. Widespread sales resumed in 2006 with FDA approval over vocal protests from consumer advocates.

“This is exactly the kind of problem we were concerned about when we said we don’t know enough about these products and whether they are safe,” said Amy Allina, policy director at the National Women’s Health Network.

An estimated 5 million to 10 million women around the world have breast implants.

The FDA said its review found about 60 cases since 1997 of anaplastic large cell lymphoma (ALCL), a type of immune-system cancer. The number is tough to verify and some reports could be duplicates, the agency said. The FDA said “women with breast implants may have a very small but increased risk of developing this disease in the scar capsule adjacent to the implant.”

“We need more data” to better understand the issue, said Dr. William Maisel, chief scientist in the FDA’s device unit.

The risk to Allergan’s implant sales is “nearly zero,” Gleacher & Co analyst Amit Hazan said in a research note.

“There are very few FDA approved products that would get a report (and media event) like this about such a small issue, but breast implants are at the top of that list,” he said.

ALCL is rare in women without implants. In the United States, the disease is found in breast tissue in about three out of every 100 million nationwide without implants.

Mentor and Allergan said they supported the FDA action and agreed the number of cases was small.

ALCL is “extremely rare and not to be mistaken for breast cancer,” Allergan spokeswoman Caroline Van Hove said.

“A woman is more likely to be struck by lightning than get this condition,” she said.

Symptoms, including persistent swelling or pain near the implant, appeared between one year and 23 years after the devices were inserted, the FDA’s Maisel said. He advised women to contact a doctor if they have symptoms. Data on treatments is limited but they may include removal of the implants, chemotherapy or radiation.

Officials do not know if women face a higher risk if they get implants for reconstruction after cancer surgery or for cosmetic reasons, Maisel said.

The agency is setting up a registry to track implants and working to add information to implant labels.

“We fully support FDA’s efforts to gather additional data and study ALCL in patients with breast implants,” Mentor spokesman Christopher Allman said.

The agency plans to release interim findings from ongoing studies of silicone implants in the spring. As a condition of approval, each maker was required to study risks in 40,000 women for 10 years.

Allergan shares fell 1.9 percent to $70.72 on the New York Stock Exchange. Johnson & Johnson shares slid 0.8 percent to $60.60, also on the NYSE.

(Additional reporting by Debra Sherman in Chicago; Editing by Lisa Von Ahn, Gerald E. McCormick, Steve Orlofsky and Carol Bishopric)

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By Lisa Richwine and Susan Heavey

WASHINGTON