TUESDAY, Dec. 5 (HealthDay News) -- The popular hair-loss drug Propecia can change the results of a common screen for prostate cancer, the prostate-specific antigen (PSA) test, leading to skewed readings that might obscure the presence of disease, a new study found.

Men using Propecia "should be aware that their PSA will be falsely lowered" and that small changes upward could be signs of trouble, said study lead author Dr. Anthony D'Amico, chief of genitourinary radiation oncology at Brigham and Women's Hospital in Boston.

The findings of the study, which was funded by Merck & Co., the maker of Propecia, were published online Dec. 5 in the journal Lancet Oncology.

Men typically begin regular PSA testing after they reach their 40s and 50s to screen for signs of prostate cancer. PSA tests reveal blood levels of prostate-specific antigen, a protein produced by the prostate gland. Men normally have low levels of PSA in their blood; however, prostate cancer or benign conditions can increase PSA levels.

Propecia's active ingredient, the drug finasteride, was first developed to treat enlarged prostates by targeting an enzyme that plays a role in causing the prostate to grow. Later, finasteride became a treatment for baldness, and about one million American men now take the drug to stave off hair loss.

In a large study released in 2003, the finasteride formulation aimed at treating enlarged prostate, Proscar, was linked to lowered rates of prostate cancer. According to D'Amico, doctors have also known for several years that Proscar can change PSA levels in men.

Experts haven't known whether Propecia might have similar effects, however, since it contains just one-fifth of the amount of finasteride as Proscar.

In the new study, researchers gave either Propecia or a placebo to 355 men between 40 and 60 years of age with male pattern baldness. Then they studied changes in the men's PSA levels. The researchers found that within 48 weeks, PSA levels dropped by 40 percent in men aged 40 to 49 who took Propecia and by 50 percent in those aged 50 to 60.

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."