This means men on Propecia may have dangerously high PSA levels even if their test scores are in the normal range, the researchers say.
It's already known that Proscar — a drug for men with enlarged prostates — affects the PSA test.
Propecia is a low-dose version of Proscar — 1 mg of finasteride in Propecia, versus 5 mg in Proscar.
Does the low-dose drug have the same PSA effect as the high-dose drug?
Yes, says a study from Anthony V. D'Amico, MD, PhD, of Brigham and Women's Hospital, and Claus G. Roehrborn, MD, of the University of Texas Southwestern Medical Center.
"We found that Propecia does exactly the same thing as Proscar — it cuts PSA in half after you use it for a year," D'Amico tells WebMD.
Unfortunately, cutting PSA in half doesn't mean that cancer risk is cut in half.
Instead, it means men on Propecia may have an unseen problem.
"The men who use Propecia are young men, in their 30s, 40s, and 50s," D'Amico says. "This is important because these are the guys who, if they have prostate cancer, need to be diagnosed."
The D'Amico/Roehrborn study appears in the early online edition of The Lancet.
PSA Screening Controversial
PSA — prostate-specific antigen — is a marker given off by prostate cells.
Prostate cancer can make PSA levels rise, although you can have cancer without an elevated PSA.
And PSA screening can catch early prostate cancer — although it can't tell whether the cancer is dangerous.
The benefit of PSA screening is catching dangerous prostate cancers early, while they're still curable.
The risk of the screening is that men who don't have dangerous cancers may get unnecessary biopsies and treatments.
Doctors hotly debate whether PSA screening is a good or bad idea for most men. Clinical trials are under way in the U.S. and Europe that may help settle the issue.
Current guidelines don't recommend PSA screening for all men. Instead, the guidelines suggest doctors discuss PSA screening and annual digital rectal exams with men 50 and older, although screening may begin at younger ages for some men.
Doctors send patients for biopsies when a man's PSA level hits a certain threshold.
While there's no PSA level threshold below which nobody has cancer and above which everybody has cancer, the conventional normal range is between 0 and 4.0.
However, for men on Propecia, it now seems, a PSA level of 2.0 could mean trouble.
"If you've been on Propecia for a year, you should double the PSA score," D'Amico says.
"If you are on Propecia for more than a year, don't multiply the score; look at the change in PSA over time. If it goes up by more than three-tenths of a point, consider a biopsy," he says.
The danger is that men may get Propecia from one doctor and PSA tests from another, says urologist Yair Lotan, MD, of the University of Texas Southwestern Medical Center.
"So if your internist doesn't know you're being treated for male pattern baldness, and your PSA is 2.5, they may assume you are normal — but you really need to double that [score] and see a urologist," Lotan tells WebMD.
It's a difficult problem, agrees Wayne B. Harris, MD, assistant professor of hematology and oncology at Emory University and the Atlanta VA Medical Center.
"The next guy you see may have a PSA of 3 that is really equivalent to a 6 — but you don't know it because you don't know he's taking Propecia," Harris tells WebMD.
Harris and Lotan urge men taking Propecia to be sure all their doctors know about it.
Does Propecia Make PSA Cancer-Specific?
Whether they favor PSA screening or not, experts agree there's a basic problem with the PSA test: It is not specific for bad cancers.
Does Propecia make the PSA test more cancer-specific? D'Amco thinks it might, although he's quick to note this is far from proven.
"What you are actually doing by lowering PSA with finasteride is lowering the benign part of the prostate," he says. "The unanswered question is whether Propecia can make PSA more cancer specific."
Until that's proven, Harris says, Propecia's effect on PSA further muddies the water of prostate cancer screening.
"It is not making the PSA test more specific, just making the PSA test more difficult to interpret," he says. "It is potentially masking the cancer."
SOURCES: D'Amico, A. and Roehrborn, C. The Lancet, early online edition, downloaded Dec. 6, 2006. Anthony V. D'Amico, MD, PhD, professor of radiation oncology, Brigham and Women's Hospital and Dana-Farber Cancer Center, Boston. Yair Lotan, MD, University of Texas Southwestern Medical Center at Dallas. Wayne B. Harris, MD, assistant professor of hematology and oncology, Emory University, Atlanta, and Atlanta VA Medical Center.
By Daniel DeNoon
Reviewed by Louise Chang