Men's health
5-alpha-reductase inhibitors
Finasteride and dutasteride — Shrink an enlarged prostate over months — a different approach from alpha-blockers.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
These treat the urinary symptoms of an enlarged prostate by reducing its size over time. They are particularly useful for larger prostates and to lower the risk of complications such as acute retention. Finasteride is also used in male-pattern hair loss.
How it works
They block the enzyme that converts testosterone into its more potent form (dihydrotestosterone), which is the main driver of prostate growth. With less of that hormone, the gland gradually shrinks — a process that takes months, which is why symptom benefit is slow to appear.
In practice
In practice these drugs actually shrink the prostate rather than just relaxing it, which makes them suited to larger glands and to reducing the longer-term risk of acute urinary retention and the need for surgery — but the benefit builds slowly over months, so they are not a quick fix and are often combined with an alpha-blocker for faster symptom relief. Two practical points matter. They lower PSA (the prostate blood test) by roughly half, so results must be interpreted with that in mind to avoid missing prostate cancer. And they are absorbed through the skin and are teratogenic, so the tablets must not be handled by women who are or could become pregnant, and broken or crushed tablets are avoided. Sexual side-effects (reduced libido or erectile difficulty) can occur and occasionally persist.
Examples
Practical use
How to take it & use it well
- Take finasteride or dutasteride once daily, with or without food, at roughly the same time each day to keep a steady habit.
- Swallow the capsule or tablet whole and do not break, crush or chew it, as the coating helps protect the medicine and the person handling it.
- Be patient, as it usually takes several months of regular use before you notice an improvement in urinary symptoms or prostate size.
- Keep taking it as prescribed even once you feel better, because benefits depend on continued use and symptoms can return if you stop.
- Pregnant women and women who could become pregnant must not handle crushed or broken tablets, as the medicine can harm a developing baby.
- Tell any clinician arranging a prostate-specific antigen (PSA) blood test that you take this medicine, as it lowers the reading.
Common uses
- Benign prostatic enlargement (especially larger glands)
- Reducing the risk of acute urinary retention/surgery
- Male-pattern hair loss (finasteride, lower dose)
Monitoring
- Symptom and prostate-size response over months
- PSA interpreted with the halving effect in mind
- Sexual side-effects and mood
Weighing it up
Advantages & disadvantages
Advantages
- Can shrink an enlarged prostate over time, easing urinary symptoms such as a weak stream and frequent need to go.
- May reduce the risk of sudden inability to pass urine and the need for prostate surgery in some men.
- Taken as a simple once-daily tablet, which is easy to fit into a routine.
- Can be combined with other prostate medicines for men who need extra symptom relief.
Disadvantages
- Works slowly, so it can take months before the full benefit is felt.
- May cause sexual side effects such as reduced sex drive, erection difficulties or changes in ejaculation in some men.
- Lowers the PSA blood test result by around half, which must be allowed for when screening for prostate problems.
- Can cause breast tenderness or enlargement, and any new breast lump or change should be reported promptly.
Key safety principles
What to watch for
- They lower PSA by about half — interpret prostate-cancer screening results accordingly.
- Teratogenic and skin-absorbed — women who are or may become pregnant must not handle the tablets, and crushed or broken tablets are avoided.
- Sexual side-effects (reduced libido, erectile dysfunction) can occur and occasionally persist; benefit builds over months.
Key interactions
What to avoid or check alongside
- These medicines are generally well tolerated alongside other treatments, but always tell your pharmacist about everything you take.
- When combined with an alpha-blocker such as tamsulosin for prostate symptoms, the alpha-blocker can add to dizziness or low blood pressure on standing.
- Other medicines that affect sexual function may add to sexual side effects, so mention any new symptoms.
- Because the medicine lowers PSA readings, tell any clinician interpreting that test so results are not misread as reassuring.
Patient & carer advice
- It works slowly over months — often combined with another drug for quicker relief
- Women who are or could be pregnant must not handle these tablets
- Tell any doctor checking your PSA that you take this
Answers
5-alpha-reductase inhibitors: frequently asked questions
How long until finasteride or dutasteride works?
These medicines act slowly. It often takes several months of daily use before urinary symptoms improve or the prostate becomes smaller, so keep taking it as prescribed.
Why must pregnant women not handle these tablets?
The medicine can affect the development of a male baby. Pregnant women, or those who might become pregnant, should not handle crushed or broken tablets, though intact coated tablets are designed to limit contact.
Does this medicine affect my PSA test?
Yes. It lowers the PSA blood test result by roughly half. Anyone interpreting your PSA needs to know you take it so the figure can be adjusted and a rising trend is not missed.
Can these medicines cause sexual side effects?
Some men notice reduced sex drive, erection problems or changes in ejaculation. These often settle, but discuss any troublesome or persistent effects with your prescriber.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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