Caffeine could one day be used to cure skin cancer, according to US scientists who have found that the stimulant combined with an extract from green tea can kill cancer in mice.
In a new study, the researchers discovered that rubbing both substances onto the skin of mice stopped the cancer from spreading and killed all tumours, reported BBC Health.
More research will be required to find out whether the technique could also benefit people.
In the study, published in the Proceedings of the National Academy of Sciences, Allan Conney and colleagues at Rutgers University in New Jersey studied a special strain of hairless mice for a period of 20 weeks.
The mice were exposed to ultraviolet B light twice a week during this time, putting them at risk of skin cancer. After the exposures, the researchers applied two components of green tea - caffeine and an extract called epigallocatechin gallate (EGCG) - to the mice's skin.
They found that both substances had successfully tackled cancer. However, they believe that caffeine may be more effective because it is more stable chemically than EGCG.
Conney said: "We may have found a safe and effective way of preventing skin cancer."
The caffeine and ECCG combination killed cancer cells without damaging any surrounding skin.
"The discovery of this selectivity was very exciting to us," said Conney. "Also in our study, it didn't matter if the tumours were benign or malignant. Cells in both were killed while leaving the normal cells alone."
Previous studies have suggested that drinking caffeine can also help to fight skin cancer. However, the high doses required may not be suitable for human consumption.
"Whether you can give enough orally to be effective in humans is not known," Conney noted. "Whether people could ingest that amount without becoming hyperactive is also a real question mark."
The research team said they were planning to carry out trials to see if caffeine had a similar impact on humans with skin cancer although they added that a caffeine treatment was a long way off.
"For now, if you are a mouse, it would be terrific. In people, we just don't know yet," Conney concluded.