Finasteride and dutasteride lower PSA by roughly half after six to twelve months of treatment. A PSA that looks normal on these drugs may therefore be falsely reassuring, and a common working rule is to double the measured value when comparing it with age-specific reference ranges.
The more useful signal is trend. Record a new baseline about six months after the drug is started and treat any sustained rise from the lowest value as significant, even if the absolute number is low. Check adherence before acting, since stopping the drug will raise PSA, and document clearly in notes and referral letters that the patient is on a 5-alpha reductase inhibitor.
- Expect PSA to fall by about half after 6 to 12 months on a 5-alpha reductase inhibitor.
- Double the measured value when comparing with age-specific ranges.
- Set a new baseline about six months after starting the drug.
- Treat a sustained rise from the nadir as significant.
- Check adherence before interpreting a rise.
Why it matters
An unadjusted PSA on these drugs can hide a rising cancer marker.
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