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All men tend to produce less of the male hormone testosterone as they grow older, but some men produce less than others, which is why there was so much interest — and concern — last week about a new study that showed that older men with particularly low levels of testosterone circulating in their blood may be at higher risk of developing Alzheimer's disease. Other studies have shown that testosterone improves performance on cognitive tests, but this is the first to suggest that low levels of circulating testosterone may provide a warning — as much as a decade in advance — that a man will develop Alzheimer's.

The study, published in the journal Neurology, tracked the testosterone levels in 574 men ages 32 to 87. After 19 years, about 1 in 10 had been diagnosed with Alzheimer's disease. Although testosterone levels were down across the board, they fell precipitously in the men who developed Alzheimer's, to about half the levels of the men who did not develop the disease. Dr. Susan Resnick, who wrote the paper, believes testosterone may interfere with the production of the plaques and tangles associated with the devastating brain disorder.

If she's right, her study is certain to increase demand for testosterone prescriptions, sales of which have shot up some 500% over the past decade. About 800,000 American men take prescription testosterone, in some cases to treat hypogonadism (abnormally low testosterone levels), in others to boost normal testosterone levels in hopes of increasing muscle mass or athletic performance.

Most doctors, acutely aware of the problems female hormone-replacement therapy caused for women, are cautious about advising anyone to take testosterone supplements — even to ward off Alzheimer's. While the pros are still being worked out, the cons are real and troubling. Resnick is convinced that testosterone supplements can increase your risk of prostate cancer and stroke, despite a report in the New England Journal of Medicine last week suggesting that the studies were not large enough to say for sure. Furthermore, the ideal levels of testosterone still need to be worked out. "An 80-year-old should probably not have the levels of a 20-year-old," says Resnick, no matter how bad his memory is.

One last word of warning: don't try getting around your doctor by buying testosterone on the Internet. A lot of products sold on the Net claim to contain testosterone-like substances, but for the real deal, you need a prescription.

Dr. Gupta is a neurosurgeon and CNN medical correspondent

• Sanjay Gupta