The warning labels are necessary because the danger is proven but is being ignored, according to Professor Gordon McVie, director general of The Cancer Research Campaign.

"My advice to smokers who can't kick the habit is don't add to your risk of getting cancer by taking beta-carotene supplements," McVie says in a statement on The Cancer Research Campaign's Web site.  "The research is cast iron in my view, and I feel certain most smokers are not aware of this risk," he adds.

Beta-carotene occurs naturally in foods. Researchers at one time thought it might protect against cancer because early studies showed that people who ate a lot of foods rich in beta-carotene and other antioxidants were less likely to develop cancer.  Antioxidants help prevent damage to DNA that can begin the process of a cell turning cancerous.

But two large studies show beta-carotene, when taken in pill form, actually increased the risk of lung cancer in smokers, McVie says.

The first study, published in 1994 in The New England Journal of Medicine, (Vol. 330, No. 15) involved over 29,000 male smokers in Finland.  Some of the men took beta-carotene while others took look-alike pills containing no beta-carotene.  Those getting the beta-carotene had 18 percent more lung cancer.

The second study, done in the United States and published in 1996 in the Journal of the National Cancer Institute (Vol. 88, No. 21), looked at lung cancer rates in 18,000 male and female smokers.  Those getting a combination of beta-carotene and vitamin A supplements had 28 percent more lung cancers and 17 percent higher death rates compared to those getting none of either supplement.

A U.S. expert on chemoprevention -- the use of substances to reduce the chances of developing cancer -- says experimental studies done later helped scientists understand why smokers taking beta-carotene developed more cancer.

"Later studies showed that beta-carotene promotes the activation of carcinogens [cancer-causing substances] in tobacco smoke, and can therefore damage more DNA," says Wuan K. Hong, M.D., an American Cancer Society clinical research professor and chairman of the department of thoracic and head and neck oncology at M.D. Anderson Cancer Center in Houston. 

Scientists know that damage to DNA by a carcinogen is often one of the first steps in a process that can change a normal cell into a cancerous cell.

A manufacturer of beta-carotene supplements was quoted on The Cancer Research Campaign's Web site as saying synthetic beta-carotene was used in those studies and perhaps results would be different if a natural form were used instead.  Dr. Hong says that cannot be known, as such studies have not been done.

"Medicine must be practiced on the basis of evidence, and the evidence shows that getting plenty of beta-carotene in foods is good," he says. "But people who are active smokers and taking beta-carotene supplements at the same time develop more lung cancer."

"I would have to agree with Professor McVie and not recommend beta-carotene supplements for individuals who continue smoking," Dr. Hong adds.