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."
The study, published in the Lancet Oncology, followed 308 men age 40 to 60 with male-pattern baldness. The data were collected by Dr. Claus Roehrborn, a urologist at the University of Texas Southwestern Medical Center while he worked with Merck on developing the drug. Merck funded the study.
For 12 months, 247 men took 1 milligram 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% 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.
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.