Hair-Loss Drug Skews Prostate Cancer Test

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By contrast, PSA levels increased an average of 13 percent in those men who took a placebo.

The findings suggest that doctors should adjust their interpretation of PSA tests for men who are taking Propecia long-term, in much the same way they now adjust for Proscar, the study authors said. But the keyword is careful observation.

"You can start by multiplying it [PSA readings] by two, but that is not the [full] answer," D'Amico said. "You have to look at the level each year. If it goes up three-tenths of a point, you should be talking about a referral for a prostate biopsy."

Why does Propecia disrupt PSA levels in the first place? "By blocking the ability of testosterone to act on the prostate gland, you shut off the enlargement of the gland," D'Amico said. "If you don't make benign prostate tissue, you don't make the PSA that's associated with it. And that takes PSA levels down."

More information

For more on prostate cancer and the PSA test, visit the U.S. National Cancer Institute (www.cancer.gov ).