Philadelphia ? Caffeine may help prevent skin cancer, according scientists at Rutgers University.

Experiments with mice showed caffeine seemed to fend off the formation of squamous cell carcinoma, a common form of skin cancer, by helping kill off precancerous cells.

But before you pour yourself a second cup of coffee or tea, you should know that the best results came not from drinking caffeine but from direct application to the skin. If it proves to work as well in humans, caffeine or some similar compound might someday be added to lotions or creams.

That could have huge benefits for the many people who roasted themselves as teenagers, an activity that dermatologists say does irreparable cell damage and leads to an elevated lifetime risk. About a million people a year are diagnosed with skin cancer.

Squamous cell carcinomas are usually curable if caught early. The researchers don’t know if caffeine has any effect on melanoma, a much rarer and deadlier form of skin cancer.

The findings were published in today’s issue of the Proceedings of the National Academy of Sciences.

Lead researcher Allan Conney, head of cancer research at the Rutgers school of pharmacology, said he became interested in caffeine while exploring possible benefits of green and black teas. Both seemed to prevent cancer in mice, he said, but decaf brews didn’t work nearly as well.

That led him to try giving the animals straight caffeine.

He started by adding caffeine to the animals’ drinking water and found it seemed to protect them from skin cancer, though the mice had to ingest much more than most people would be able to tolerate.

So he tried a topical approach. He used specially bred hairless mice and exposed them to ultraviolet light to put them at risk for squamous cell skin cancer. After he stopped the ultraviolet exposure, he left one group of mice untreated while the other got daily skin applications of a caffeine solution, about a drop a day. After about 20 weeks, the caffeine-treated mice had about 70 percent fewer malignancies.

The caffeine seemed to help kill off tumor cells, Conney said, suggesting that it might work by attacking tumors while they were still microscopic.