From the FDA Drug Label
In clinical trials, dutasteride reduced serum PSA concentration by approximately 50% within 3 to 6 months of treatment. To interpret serial PSAs in men taking dutasteride, a new PSA baseline should be established at least 3 months after starting treatment and PSA monitored periodically thereafter
The information provided is for dutasteride, not finasteride. However, since both are 5 alpha-reductase inhibitors, similar principles may apply.
- PSA monitoring: A new PSA baseline should be established at least 3 months after starting treatment with a 5 alpha-reductase inhibitor, and PSA should be monitored periodically thereafter 1.
- Interpretation of PSA results: Any confirmed increase from the lowest PSA value while on treatment may signal the presence of prostate cancer and should be evaluated. Note that the provided label does not directly address finasteride, but based on the class of medication, similar guidelines might be considered.
From the Research
For patients taking finasteride consistently, PSA levels should be checked after 6 months of therapy and then annually thereafter, as the most recent and highest quality study suggests that PSA levels decrease by approximately 40% during finasteride therapy 2. After starting finasteride, PSA levels typically decrease by approximately 40-50% within 6-12 months, as shown in studies 3, 2. This baseline reduction is important to establish, as any future rise in PSA from this new baseline may indicate potential prostate cancer development. When interpreting PSA results for patients on finasteride, healthcare providers should consider the change in PSA levels over time, rather than a single value, as the risk of cancer is low if the PSA falls by > or = 50% and raised if it falls by < 33% 3. Some studies suggest that doubling the measured PSA value may be useful to compare it to normal reference ranges for men not taking the medication 3, but this approach may not be accurate for all patients, particularly those with a baseline PSA level less than 4 ng/mL 2. If there is a significant rise in PSA (even from the lower baseline) or if the PSA fails to decrease as expected after starting finasteride, further evaluation including possible prostate biopsy may be warranted. This monitoring schedule is important because finasteride masks PSA levels by inhibiting the conversion of testosterone to dihydrotestosterone, which affects prostate tissue and PSA production, but doesn't prevent prostate cancer development. Key factors to consider when evaluating PSA results in patients taking finasteride include: