The hair-loss drug Propecia interferes with the most commonly used test for prostate cancer, causing inaccurate readings that can mask the presence of the disease, researchers reported Monday.
About 4 million men worldwide use the drug, whose active ingredient is finasteride, which prevents the breakdown of testosterone. Researchers knew 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 USC who has worked on previous studies on finasteride and spoken for the drug's maker, Merck & Co., on other drugs, but was not involved with this study.
"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."
