As little as 1 mg/day of finasteride can seriously distort results of prostate-specific antigen (PSA) testing, a study shows.

That’s the dosage of Propecia used by more than 4 million men to treat male-pattern baldness, and shown by the study to lower serum PSA concentrations by as much as half. Finasteride is also sold as Proscar for BPH, but Proscar was not tested in this study.

The finding suggests clinicians should routinely ask about Propecia use when taking a prescreening history, says investigator Anthony V. D’Amico, MD, PhD, an oncologist at the Dana-Farber Cancer Institute in Boston. To compensate for the artificially lower PSA levels in patients taking Propecia, “the PSA level needs to be multiplied by two in order to assess whether a prostate biopsy is indicated or not.”

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Finasteride 5 mg/day (Proscar) is used to treat BPH, and current guidelines for patients taking the drug recommend a prostate biopsy if the PSA level is elevated by 0.3 ng/mL in one year, he added.

Researchers tested the effect of finasteride 1 mg/day and placebo on PSA levels in 355 men for 48 weeks. Among those taking finasteride, men in their 40s showed a 40% median decrease in serum PSA at the end of the study, the authors reported in Lancet Oncology(2007;8:21-25.) Those aged 50-60 years showed a 50% decline. Among placebo recipients, those in their 40s had no change in serum PSA levels whereas those aged 50-60 years had a median 13% increase. The increase in the older group was expected because PSA levels rise with BPH, and the risk for BPH rises with age, Dr. D’Amico noted.