The Food and Drug Administration Thursday issued a public health advisory, telling doctors to be careful in prescribing two skin care treatments because they have a potential cancer risk.

The announcement affects Elidel, from

Novartis

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, and Protopic, from Fujisawa Pharmaceutical. Both are prescribed for eczema, which is characterized by red, dry and scaly skin.

In addition, the FDA said the drugs will have black box warnings, the toughest restriction the agency can place on a label. The agency acted less than four weeks after an FDA advisory committee recommended extra precautions.

"It may take human studies of 10 years or longer to determine if use of Elidel or Protopic is linked to cancer," the FDA said. "In the meantime, this risk is uncertain, and FDA advises that Elidel and Protopic should be used only as labeled, for patients who have failed treatment with other therapies."

Protopic was approved in December 2000 and Elidel in December 2001. Since their approval, FDA said it has received reports of lymphoma and skin cancer in children and adults. "Whether the reported cancers are associated with these products has not been clearly established," the agency said.

The advisory committee sounded an alert because of these reports and because tests showed the cancer risk in mice, rats and monkeys rose as the dose and duration of use increased.

The drugs aren't approved for children under 2 years old. They are prescribed for short-term and intermittent treatment of patients who don't respond to other eczema therapies.

"Elidel and Protopic should be used only for short periods of time, not continuously," the FDA said. "The long-term safety of these products is unknown."