Finasteride: what it does and what to watch
Also known as: 5-alpha reductase inhibitor, DHT-blocking oral medicine · 5-alpha reductase inhibitor · Updated 2026-08-20 · Educational — not medical advice
In plain language: This medicine blocks the enzyme that converts testosterone into dihydrotestosterone (DHT), a more potent hormone linked to shrinking hair follicles in genetically susceptible scalp areas and to prostate gland enlargement. By lowering DHT levels, it can slow or partially reverse hair thinning in some people and shrink prostate tissue over time, though effects build gradually over months.
What it's used for
- Male pattern hair loss
- Symptoms of an enlarged prostate gland (benign prostatic enlargement)
Common side effects
- Decreased sex drive
- Difficulty achieving or maintaining an erection
- Reduced ejaculate volume
- Mild breast tenderness or swelling
- Dizziness
Get medical help if…
Sexual side effects that persist for some people even after stopping the medicine, though this appears uncommon and is still being studied
New or worsening low mood or depression
Breast lumps, pain, or nipple discharge, which should be evaluated promptly since they can rarely reflect breast tissue changes
Signs of an allergic reaction, such as swelling of the lips, face, or difficulty breathing
Testicular pain
Lab values it can move
Few guides tell you this part: medicines show up in blood work. If you take Finasteride and a value below looks off on your report, this may be part of the story — never stop a medicine without your doctor.
This medicine roughly halves PSA readings even when no prostate problem is present, so a prescriber typically doubles the measured value when interpreting PSA results for prostate cancer screening in someone taking it.
PSA (Prostate-Specific Antigen)Because this medicine blocks conversion of testosterone to a more potent form, circulating testosterone itself may rise slightly while its more active downstream hormone is reduced.
Testosterone, TotalGood to know
- People who are pregnant or who could become pregnant should not handle crushed or broken tablets, since the medicine can be absorbed through the skin and may harm a developing male fetus
- Tell any doctor ordering a PSA test that you take this medicine, since the reading needs to be adjusted to interpret it accurately
- Discuss any sexual side effects honestly with a prescriber rather than stopping the medicine abruptly on your own, since alternatives or adjustments may help
- Report new breast changes, lumps, or discharge promptly
- Hair benefits generally fade within months of stopping the medicine
Questions people ask
Why shouldn't a pregnant woman even touch these tablets?
This medicine can be absorbed through the skin from crushed or broken tablets, and even small amounts of the active ingredient can interfere with normal development of a male fetus's genitals if a pregnant woman is exposed. Whole, coated tablets handled normally pose far less risk, but crushing or breaking them should be avoided by anyone who is or might be pregnant.
Will this medicine mess up my prostate cancer screening results?
Yes, in a predictable way. This medicine typically reduces PSA levels by about half after a few months of use, regardless of whether a prostate problem is present. A prescriber accounts for this by roughly doubling the measured PSA value when assessing prostate cancer risk in someone taking it, so it's important to mention this medicine before any PSA test.
I'm having sexual side effects, should I just stop taking it?
Sexual side effects such as reduced libido or difficulty with erections are a recognized possibility with this medicine and are worth discussing openly with a prescriber rather than stopping abruptly on your own. For most people these effects ease if the medicine is stopped, and a prescriber can help weigh alternatives or adjustments based on your specific situation.
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