Several creams caused skin cancer in the specially bred mice, which had been pre-treated with ultraviolet radiation, researchers found.

The scientists stressed in their report that the cancers are not melanoma, the deadliest kind of skin cancer, but another type called squamous cell carcinoma.

Such tumors are slow growing, highly treatable and only fatal if patients fail to have them removed.

The discovery was noticed while testing a theory that caffeine could prevent skin cancer.

"We sort of got into this by accident," said Allan Conney, who led the experiment with colleagues at Rutgers University in New Jersey. "We wanted a safe cream that we could put the caffeine into."

The team used specially bred albino mice, which are prone to develop skin cancer. The mice are pre-treated with ultraviolet radiation to simulate the effects of a human who has had heavy sun exposure in the past but then stopped -- something that may be happening in the population as people realize the risks of getting a tan.

They decided to test the creams first and found that all four they tested caused tumors to grow on the mice.

Conney said he does not know why but suspects two ingredients -- mineral oil and sodium laurel sulfate.

"We'd like to understand the mechanism. What is most important is to see whether these moisturizing creams are tumorigenic in people," Conney said.

Conney said his lab was not equipped to continue further experimentation, but suggested others should.

But he added, "I don't think that people should be scared because this is a mouse study and we don't know whether or not it applies to humans."

Other experts were skeptical about the study.

"This is a pretty artificial situation with the mouse skin primed by a lot of UV light to develop cancer," said Karol Sikora, Medical Director of Cancer Partners U.K. in Britain.

Conney and colleagues went to Johnson & Johnson to create a cream without the suspect products and they developed one using other readily available ingredients. "They are things that are commonly used in many moisturizing creams," he said.

They said the newer cream did not cause the tumors in the mice.

Rutgers and Johnson & Johnson had patented the new cream jointly but did not know whether it would be commercially developed. The company has yet to comment.

The team’s report can be found in the Journal of Investigative Dermatology.

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