By now I am certain that the most complicated piece of equipment I operate on a daily basis is not my computer. It's my body. As the sole certified owner, I am expected to download the latest information to my user's manual and then promptly update my lifestyle, my lunch and my medicine cabinet. But sometimes it's enough to make my hard drive crash.

This ungrateful thought about modern medical science was surely fueled by the recent rash of revisionist research on calcium, low-fat diets and that old favorite, estrogen. But it was finally ignited by a study on breast cancer prevention.

My morning began with the good-news bulletin — "Breakthrough in Breast Cancer" — about a study of nearly 20,000 women testing one of two drugs, tamoxifen and raloxifene, for the prevention of breast cancer. Raloxifene seemed to offer a better new choice for the 9 million post-menopausal women at higher-than-average risk for the disease.

As one of those 9 million women, I promptly called another of the 9 million with the question, "OK, what do we do now?" And here is where the breakthrough became a breakdown.

Tamoxifen, a drug used to treat cancer, had also been hailed as a breakthrough some years ago when it was found to cut the risk of invasive cancer in half. But it came with some pretty scary risks, from uterine cancer to blood clots. Raloxifene appeared to have the same benefits with fewer risks. But how many fewer risks? For whom? For how long? Sometimes risk-benefit analysis comes with a special side effect that in the medical literature is technically known as "driving us nuts."

Jerome Groopman, an oncologist and author of "The Anatomy of Hope," wryly imagines a different headline: "There's a Small Difference in a Larger Group of Women That Has Side Effects and It's Not Clear What's Best for Any Individual." Not sexy, but pretty accurate.

For openers, the side effects of uterine cancer and blood clots were lower with raloxifene, but not by much. We don't yet know whether either drug prevents or just delays breast cancer. Meanwhile, raloxifene, the "winner" in this face-off, didn't cut the risk of those early noninvasive tumors as well as tamoxifen, the "loser." And while your head is spinning, remember that both drugs are likely to send women back to those wonderful yesteryears of hot flashes and night sweats.

When Cynthia Pearson of the National Women's Health Network does the math, she figures that only 30 of the 10,000 women who took raloxifene for up to five years actually benefited, once the serious risks are taken into account. This may be good for some women, she says, but it "isn't ready for prime time."

Here is where the cancer gnaws. And the heart disease. And the high blood pressure. And the Alzheimer's. And whatever rises to the top of the list of risks and anxieties in your owner's manual.

In the bad old days, doctors were the sole proprietors of medical knowledge — which they doled out sparsely. Today we are all on the receiving end of an open fire hose of information.

In the bad old days, patients put themselves in the hands of doctors. Now consumers are more likely to believe that our health is in our hands. With that comes the belief that we must do our own statistical analysis, risk-benefit assessment and double-blind studies of one.

This shift in information and decision-making was supposed to be empowering. It often is. But the flip side is that we can also end up feeling confused and wholly responsible.

How many of us have sat with people we love as they face an array of conflicting and often grim treatment options? As Harvard Medical School's Lachlan Forrow says, "When we give you choices, it feels like you have control over the outcome and responsibility for choosing the right outcome. It puts an incredible burden on you." The added burden is that even death begins to seem like the result of a bad choice.

"OK, what do we do now?" I've done my high-risk assessment — factored in a mother and aunt, breast cancer survivors at 86 and 91 — and come up with a relatively easy answer: Wait. But in the next months, millions of women will be making their own decisions. How many will share the same worries: If we reject a drug that might prevent breast cancer, are we asking for it? If we take a drug with side effects, are we choosing them?

Such queasy questions are now a permanent feature of our owner's manual. Until, that is, we finally get a "breakthrough" that looks a lot more like the real thing.