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Finasteride blocks the conversion of testosterone into dihydrotestosterone (DHT), a leading cause of hair loss. Therefore, since less testosterone is being converted, serum levels of testosterone do increase in patients taking finasteride. The biggest increases in testosterone levels tend to be in patients who have low testosterone levels to begin with. In many cases, testosterone levels will drift back towards original levels with more time taking the medication.

Finasteride also affect PSA levels, which is the main test done to detect prostate cancer. Finasteride will lower PSA levels, which makes it more difficult to detect certain prostate cancers until they are in later stages. Therefore, it is always important to inform your urologist that you are taking finasteride.