LOS ANGELES, Dec. 5 (Xinhua) -- The hair-loss drug Propecia can skew the most commonly used test for prostate cancer, according to a study.

    The medicine causes inaccurate readings that can mask the presence of the disease, said the study that appeared on the Los Angeles Times website on Tuesday.

    The drug's active ingredient finasteride prevents the breakdown of testosterone. Researchers found that the high doses of finasteride in Proscar, the drug used to treat an enlarged prostate, could reduce levels of the marker called prostate specific antigen, or PSA.

    The new study is the first to show that the lower levels in Propecia also lower PSA levels.

    The suppression could mean that a previously safe reading on a PSA test could be false, said Dr. David Quinn, an oncologist at University of Southern California (USC) who has worked on previous studies on finasteride.

    "If people are taking Propecia, they should know the PSA may not be the most accurate predictor of cancer," he said. "Other tests, such as the digital rectal exam, where the doctor is feeling the prostate, may be of more importance."

    The study followed 308 men age 40 to 60 with male-pattern baldness. For 12 months, 247 men took 1 mg of finasteride daily, while 61 took a placebo. In men taking Propecia, PSA readings steadily declined. Compared with the beginning of the study, the readings at the end were a median of 48 percent lower. The effect was slightly greater on men in the 50 to 60 age group.

    "Doctors often tell men that their PSA is 'normal,' but don't tell them what the number is," he said. "They should always know what the number is."

    Roehrborn said other drugs in this class, known as 5-alpha reductase inhibitors, will affect PSA tests in a similar way.

    About 4 million men worldwide use the baldness drug.

    Prostate cancer is the most commonly diagnosed cancer in men, with about 230,000 new cases expected in 2006, according to the American Cancer Society.