The US Food and Drug Administration (FDA) is warning eczema patients and their doctors that the eczema treatments Elidel (pimecrolimus) and Protopic (tacrolimus) may carry a risk of cancer.

The drugs have been shown to raise the risk of skin cancers and lymphoma in animal studies, the agency said. There have also been a few reports of these cancers in children and adults using the medications.

It's not certain that the medications are responsible for the cancers. The FDA said long-term human studies would be needed to know for sure. Until such studies are done, though, the agency said the two drugs should be used only after other treatments have failed, and then used only as directed.

Novartis, which manufactures Elidel, and Fujisawa Healthcare, which makes Protopic, have agreed to conduct these studies, the FDA said. The companies must also add a black box warning to the drugs. These warnings alert doctors to potentially serious side effects of a drug.

Elidel and Protopic are approved for use only for short periods of time and only in patients 2 years old or older.

The drugs control eczema by suppressing the patient's immune system. It is known that people with a defective immune system are more prone to develop cancers, particularly lymphomas.

In animal studies, higher doses of the drugs were associated with higher risk of cancer, the FDA said. In addition, skin tumors formed faster when the treated animals were exposed to sunlight.

For these reasons, the FDA said patients should use as little of the creams as possible and limit their exposure to the sun, sun lamps, and tanning booths while using the drugs.