An anti-viral drug used to treat genital herpes and facial blisters can help skiers avoid an outbreak of cold sores, said Dr. Spotswood Spruance, a University of Utah infectious diseases professor.

Spruance, who studied skiers at Snowbird and Steamboat Springs, Colo., said that people with a history of cold sores triggered by the sun were 75 percent less likely to develop sores if they took the drug, acyclovir, before hitting the slopes.Doctors believe exposure to high levels of ultraviolet light during alpine sports may trigger the sores, caused by the virus Herpes simplex labialis. The blisters may appear on the lips, chin, nose, tongue or cheeks, and usually last about a week.

Spruance headed the study, which involved 147 cold-sore sufferers, many of whom were physicians attending medical conferences in Utah and Colorado.

The researchers found that only 7 percent of those who took 200 milligram acyclovir tablets twice a day for seven days still came down with cold sores, while 26 percent of those given sugar-pill placebos developed sores. Both groups began taking pills 12 hours before their first sun exposure.

"We found those who were taking the drug were very successful in blocking the development of lesions," Spruance said.

"Since we have trouble in the treatment of herpes blisters, prevention is a more reasonable way to go about treating selected patients - as long as you know when it is you're likely to get one," he said.

"We seem to be making some headway in drugs for the treatment of the disease, and acyclovir does seem to provide some measure of protection. But I also want to point out that this does not mean you shouldn't continue to use sunscreen, because anti-virus pills don't do anything about the other long-term effects of excessive ultraviolet exposure such as wrinkling and a propensity for cancer," he said.