Genetic testing will determine our predisposition to a particular illness. We will take pills to lower the risk. But the pills may have side effects. And there may be pressure to take the preventive medicine even if we don't want to.

It's 2020. You're perfectly healthy, but you take three pills a day to prevent some loathsome disease. You will take them for years, maybe decades, even though the pills are expensive and have unpleasant side effects. There is no guarantee the pills will prevent the disease. What's more, there's no certainty you will get the disease if you don't take the pills.

Kind of a drag, huh? You have fallen into this pill-popping purgatory because genetic tests show you have a higher-than-average likelihood of developing this particular disease. The pills could help improve your odds.

Think of it as the medicalization of daily life. In the 21st century, the mere risk of disease will be treated as a disease: something to be carefully monitored and corrected. The emerging field of chemoprevention is already developing pills to ward off a range of diseases, from various kinds of cancer to diabetes to osteoporosis. The pill takers will be those who have a genetic predisposition to an illness or have other risk factors, such as a weakened immune system or exposure to carcinogens.

HEAD TO HEAD. The chemoprevention principle has already been proven: Recent clinical trials have revealed that two estrogen-like drugs, tamoxifen and raloxifene, reduce the chances of breast cancer in high-risk women by more than 50%. The American Society of Clinical Oncology now recommends daily doses of tamoxifen for any woman over 35 whose chance of getting breast cancer in the next five years is 1.7% or higher. (The calculation is based on family history, reproductive history, and the number of biopsies.) And the National Cancer Institutes is just starting the biggest chemoprevention study ever, enrolling 55,000 women to compare tamoxifen and raloxifene head to head.

But is there such a thing as being too cautious? Preventive methods, after all, can carry their own problems. Tamoxifen taken over time, for example, slightly increases the risk of endometrial cancer and heart disease. ''To give such a powerful drug to healthy women with a relatively low risk is simply too dangerous,'' complains Helen Schiff, a breast-cancer patient advocate.

Yet patients could find themselves losing control over such decisions. It's easy to imagine that insurance carriers would require high-risk customers to take preventive medicine, whether they want to or not. At the other extreme, will health maintenance organizations refuse to pay for the pills if they don't think the patient's risk is high enough? Will mortgage companies refuse to lend to high-risk patients who don't take pills? Will employers refuse to hire them?

Those questions presume that insurers and employers know about your elevated risk. They won't if genetic-test results are kept private --as many physicians hope. ''Otherwise, this could be a huge, huge problem,'' warns Dr. Raymond N. DuBois, director of the cancer prevention program at Vanderbilt University in Nashville.

Even if no one knows about your genetic Achilles' heel but you, there are still issues to face. Will people engage in high-risk behavior --smoking, for example--if they know there's a pill to prevent lung cancer? There are indications they might: Studies have shown that people who drive sport-utility vehicles are sometimes more reckless than those in more vulnerable cars--because they think they are safer.

Doctors in the field argue that chemoprevention should be thought of as just another way to stay healthy, like exercising and eating right. ''Why is it so bad to offer another option?'' asks Dr. Alexandra S. Heerdt, director of the special surveillance breast program at Memorial Sloan-Kettering Cancer Center in New York.

Plenty of such options are certain to come along. ''Given the rapidly growing understanding of the biological underpinnings of disease, I expect to see much more effective agents in the next two decades,'' says Dr. Victor G. Vogel, a professor of medicine and epidemiology at the University of Pittsburgh School of Medicine. ''It's just a matter of finding the better agents and how to administer them. That is certainly going to happen.''

TAKE AN ASPIRIN. In fact, it's already happening. The next likely prevention candidate after breast cancer is colon cancer, which killed almost 48,000 Americans in 1998. Several studies indicate that taking aspirin or ibuprofen cuts the risk of developing colon cancer in half, and clinical trials are under way to test the newer COX-2 drugs against the disease.

The NCI is also conducting a clinical trial to see if Merck's drug finasteride, now used to treat prostate cancer, can also be used to prevent it. For people at risk of developing insulin-dependent diabetes, there is a federal study to see if giving insulin to symptomless people can delay the onset of the disease. And genisten, a component of soy, is being studied as a lung cancer preventative.

Chemoprevention raises difficult questions. How do you decide between living with poor odds and dealing with the costs, hassle, and possible side effects of long-term pill usage? In the next century, it could well be the biggest decision you'll make about your health.

By Catherine Arnst

To read a correction/clarification about this story, click here.

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