A popular baldness drug taken by more than 4 million men worldwide can mask an important marker used in screening tests to detect prostate cancer, scientists said on Monday.

Finasteride, which is made by Merck & Co Inc. and sold under the trade name Propecia, is a leading drug to treat male-pattern baldness.

But researchers have discovered it artificially lowers a protein called prostate specific antigen (PSA). High levels of PSA in the blood can signal prostate cancer or other problems.

Dr. Anthony D’Amico, the lead author of the study from Brigham and Women’s Hospital in Boston, recommends that middle-aged men taking Propecia have their PSA levels multiplied by two in tests to account for the difference.

“The main finding is that this drug called Propecia, which men use for hair loss typically between the ages of 30-60, affects the PSA levels,” he said in an interview.

“If you are using the PSA test in order to screen men, especially young men, for prostate cancer you can be falsely misled by the lower level of PSA when men are taking this drug for hair loss,” he added. The findings are particularly important for younger men who are the most likely group to use the drug.

Merck said in a statement that the study results were first presented in 2000 and are “consistent with information that has been included in the label for Propecia (finasteride 1 mg) since the product was first approved in 1997.”

The company said information advising patients who undergo a serum PSA test to tell their doctor if they are taking Propecia has been included for consumers since initial product approval.

Prostate cancer is one of the most common cancers in men. Each year 543,000 new cases are reported worldwide and the disease kills 200,000 mostly older men in developed countries.

In an earlier trial of a drug called Proscar, which contains a higher dose of finasteride, PSA levels were also lowered as well as the risk of prostate cancer. But men taking Proscar who developed cancer had a more aggressive disease.

“The reason that probably happened is that the multiplication factor of two to correct the PSA is inadequate and as men are on the drug longer that factor needed to be increased,” D’ Amico added.

Proscar contains 5 milligrams (mg) of finasteride while Propecia has 1 mg.

D’Amico and Dr. Claus Roehrborn, of the University of Texas Southwestern Medical Center in Dallas, studied the impact on PSA levels on 355 men ages 40 to 60 years old who took Propecia for one year. The research was funded by Merck and the findings are reported in the journal Lancet Oncology.