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Caffeine, the chemical stimulant in coffee and tea, has been found to lower the risk of skin cancer in laboratory mice.

A study suggests that a skin lotion spiked with caffeine or with another compound found in green tea can reduce by more than half the number of cancer tumors on the skin of hairless mice exposed to brutal levels of ultraviolet radiation, said Dr. Allan Conney, a professor of cancer and leukemia research at Rutgers University in New Brunswick, N.J.

"We had between 50 to 70 percent tumor formation inhibition in the mice that were treated with caffeine or with EGCG (the other chemical compound)," said Conney, senior author of a study appearing this week in the online site of the Proceedings of the National Academy of Sciences.

Skin cancer is the most common of all cancers in the United States. The American Academy of Dermatology estimates about a million cases will be diagnosed in the country this year. Among them will be more than 88,000 new cases of melanoma, the disease's deadliest form. Skin cancer generally is curable by cutting, burning or freezing the tumor cells, but untreated it can be deadly.

To test effects of caffeine on skin cancer, Conney and his colleagues exposed 90 mice to high levels of ultraviolet radiation twice a day for 20 days. They used a strain of animals, called hairless mice, commonly used for skin cancer studies.

After the mice got their UVB doses, the animals were divided into three groups. One group were slathered daily with a solution of acetone and caffeine. Another group received acetone and EGCG. The third group got skin applications of acetone only. Acetone is an organic solution often used on the skin.

At the end of 18 weeks, the three groups of mice were killed, and the level of skin tumor formation was analyzed.

Conney said mice in all three groups developed malignant skin tumors, called squamous cell carcinomas, but the number of tumors per mouse was reduced by 72 percent in those treated with caffeine and by 66 percent among those treated with EGCG, compared to the controls treated only with acetone.

The treated mice also had fewer nonmalignant, sunlight-related tumors, said Conney. Compared to the control group of mice, the mice treated with caffeine had 44 percent fewer nonmalignant tumors, the EGCG group 55 percent fewer, he said.

Conney said that although both compounds were effective in lower tumor risk, caffeine has an advantage because it is chemically more stable than EGCG.

Unlike sun screen lotions, which protect against skin cancer by preventing the skin from absorbing ultraviolet rays from the sun, the caffeine's cancer protection works in the cells after exposure to the ultraviolet rays. Rays from the sun can cause genetic changes in the skin that can lead to skin cancer. Conney said caffeine apparently blocks this action by causing abnormal cells to kill themselves, a type of programed cell suicide that prevents development of abnormal growths.

"This is not a sunscreening effect," said Conney. "It is a biological effect."

He said the caffeine acts selectively, causing the abnormal cells to die but not affecting the normal cells.

Caffeine, heavily consumed in coffee, tea and some cola drinks, has been shown in other studies to prompt mental alertness in many people. Some studies have suggested caffeine aggravates symptoms of menopause or intensifies the side effects of some antibiotics. Heavy caffeine use has been linked to miscarriage. Some studies also have suggested that some people can become addicted to caffeine and can experience headaches and other symptoms when deprived of their morning coffee or cola.

Dr. Darrell Rigell, a professor of dermatology at New York University and an expert spokesman for the American Academy of Dermatology, said research like Conney's is needed badly because "skin cancer is a major problem. I hope this treatment can prove itself, because there are more skin cancers than all other cancers combined in the U.S."

He said there is a need for a "morning-after" treatment for skin cancer, a therapy that would reduce cancer risk after excessive sun exposure.

Rigell said that although hairless mice are commonly used for such research, "there is really no good animal model for skin cancer. The hairless mouse is the best of a bunch of bad choices" for testing skin cancer compounds in the laboratory.

As a result, he said, "a lot of things that work in mice cannot be extrapolated to humans."

He said other treatments that showed promise in mice have often failed when tried on humans.

Conney said the next step in studying the topical effects of caffeine will be to use the solution on people who are highly susceptible to skin cancer * people who have a precancerous condition or who already have had skin cancer.