Doctors in the not-too-distant future could be practicing chemo-prevention, the prevention of cancer by a wide array of substances.

Scientists announced last week that a chemical cousin to vitamin A -- an anti-acne drug marketed as Accutane -- can help prevent the development of additional tumors in cancer patients.

More than 30 studies are now looking at the anti-cancer effects of everything from common minerals, such as calcium, to the drug tamoxifen, under study in England for preventing breast cancer in women at high risk for the disease.

"As the science becomes better, as health care costs soar, the pressure for chemoprevention will increase enormously both scientifically and from society," said Frank L. Meyskens Jr., director of the Clinical Cancer Center at the University of California at Irvine.

Chemoprevention is based on the notion that virtually everyone comes into contact with a wide variety of cancer-causing agents, found in everything from food and water to air and sunlight. Eliminating exposure to these carcinogens is practically impossible.

Millions of Americans also engage in risky habits such as smoking or drinking too much alcohol, raising their odds of developing cancer. In addition, many people carry genes that under the right condition make them more likely to develop cancer. The combination -- exposure to cancer-causing agents in daily life plus a genetic susceptibility to the disease -- places individuals at particularly high risk of cancer.

In the United States, for example, one individual in four will develop cancer some time in their lives -- a figure that medical experts expect to increase as the population ages. "We are all at high risk," said Scott M. Lippman, assistant professor of medicine and medical oncology at the M.D. Anderson Cancer Center in Houston.

The goal of chemoprevention is to use chemicals to interfere with the two-stage cancer process, and either prevent cancer before it occurs or stop the process from recurring. For years, there have been tantalizing clues that certain substances might be able to nip cancer in the bud.

Last week, researchers including Lippman, led by Waun Ki Hong at Houston's M.D. Anderson Cancer Center, announced that they had hit pay dirt.

In a study published in the New England Journal of Medicine, they showed that high doses of Accutane -- a chemical cousin of vitamin A -- could help prevent new tumors from recurring in patients who had experienced one bout of cancer of the larynx, pharynx or the mouth and thus were at great risk of developing cancer of the lung, throat or mouth.

"This study is significant in that it is the first one that shows a lower frequency of cancer {from using chemoprevention}, and that is an important step," said Peter Greenwald, the National Cancer Institute's director of cancer prevention and control.

All 103 participants had been treated for cancer and were free of the disease when the research began. Half got Accutane daily, while the other half received a placebo or sugar pill.

The researchers found no differences in the recurrence of their original disease between the two groups. But those who got Accutane had a strikingly lower incidence of secondary tumors -- malignancies that spread to other parts of the head and neck and to the lungs. Over almost three years, only 4 percent of the Accutane group -- just two patients -- developed secondary tumors, compared with 24 percent -- 12 patients -- of the placebo group.

Hailing this as a landmark study, cancer researchers said the findings could open a new chapter in the fight against cancer. Researchers will now plan "the next generation of big studies in chemoprevention," said Meyskens. "It's likely that medicine will see a new field of prevention evolve in the next decade or so."

But researchers also cautioned that the findings should not be interpreted by the public as a license to take massive quantities of vitamin A or large amounts of Accutane, both of which can be very toxic.

Accutane "is not a vitamin," Greenwald said. "It is a synthetic analog of vitamin A, a chemical cousin, and there are a lot of side effects from taking it."

Among them are very dry and peeling skin, high levels of fats in the blood and severely chapped lips. But the biggest danger of Accutane, which is available only by prescription, are severe birth defects when the drug is taken by pregnant women.

Nor do these new findings answer a recent debate about Recommended Dietary Allowances and the role of vitamin A in the diet, researchers said. RDAs are levels of 18 essential minerals and vitamins. The National Academy of Sciences issues new levels about every five years. These nutritional recommendations serve as a gold standard for what Americans should consume each day, and in recent years there's been a great deal of controversy about levels for vitamins A and C.

"There is no firm scientific evidence that taking vitamins lowers your risk of cancer," said E. Robert Greenberg, professor of community and family medicine at the Dartmouth Medical School and co-author of another chemoprevention study. "The RDAs have to be based on firm scientific knowledge."

Taking too high a dose of vitamin A also produces toxic side effects. "People shouldn't take large amounts," Greenberg said. "They have a much greater chance of becoming sick from the vitamin A than they would from cancer."

The challenge facing chemoprevention researchers is to find substances that can prevent cancer without causing harmful side effects. Working with animals, the NCI's Michael Sporn, one of the early pioneers of chemoprevention research, has developed an analog of vitamin A that is not absorbed by the liver and so can be given at higher doses. No research has been done yet in humans.

One danger of chemoprevention is that it may give consumers the false security that they can still engage in dangerous behaviors, such as smoking, and simply pop a pill to prevent the consequences.

"I think that most of us in this field believe that it would be unwise for people to take vitamins in an effort to prevent cancer," Greenberg said. "It would help them to stop smoking and to eat a very balanced diet."

Chemoprevention is also an extremely complex new science, one which has many unanswered questions. Its complexity was highlighted by another study published last week by Greenberg and his colleagues, also in the New England Journal. That report, a paper on the effects of using beta carotene -- another chemical analog of vitamin A -- to prevent skin cancer, found no beneficial effects.

"Why these opposite results?" asks the University of California's Meyskens in an editorial accompanying the studies. The most obvious explanation, he writes, is that the beta carotene and Accutane may stop tumor formation in different ways. But because other studies show that beta carotene can reverse another type of precancerous condition in humans, "other explanations should be sought," he said.

In the meantime, the best that consumers can do is adhere to the prudent advice of following a well-balanced diet. "With regard to beta carotene, the evidence isn't there that it does anything to prevent human cancer," Greenberg said, "but there is evidence that eating a diet rich in vegetables and fruits helps."