Finasteride and dutasteride lower serum PSA by roughly half after about six months of treatment. A PSA that looks normal in a man on these drugs may therefore hide a rising true value. The common rule of thumb is to double the measured PSA to estimate the untreated level, and more importantly, to treat any confirmed rise from the on-treatment nadir as suspicious. Record whether a man is taking one of these drugs when interpreting PSA, and remember they are also prescribed for hair loss, so ask specifically.
- Ask every man having a PSA test whether he takes finasteride or dutasteride, including for hair loss.
- Roughly double the PSA after six months on treatment to estimate the untreated value.
- Investigate any confirmed rise from the on-treatment nadir.
- Record a baseline PSA before starting a 5-alpha reductase inhibitor where possible.
Why it matters
A falsely reassuring PSA is a common route to a delayed prostate cancer diagnosis.
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