Does finasteride shrink the prostate? A detailed look at its effects on BPH

October 7, 2025 •

5 min read

Benign prostatic hyperplasia (BPH), a non-cancerous enlargement of the prostate, is a prevalent condition affecting millions of men, with rates increasing significantly with age. For men experiencing bothersome urinary symptoms, the question 'Does finasteride shrink the prostate?' is highly relevant, and the answer is a definitive yes. This article delves into the mechanism, effectiveness, and considerations surrounding finasteride's use for prostate health.

Quick Summary

Finasteride is a 5-alpha reductase inhibitor that shrinks the prostate by reducing levels of the hormone dihydrotestosterone (DHT). This reduction helps alleviate lower urinary tract symptoms associated with benign prostatic hyperplasia (BPH) and can decrease the risk of urinary retention and surgery, providing long-term symptomatic relief.

Key Points

Mechanism of Action: Finasteride works by inhibiting the enzyme 5-alpha reductase, which reduces the levels of dihydrotestosterone (DHT), the hormone responsible for prostate growth.
Proven Shrinkage: Clinical studies have shown that finasteride consistently reduces prostate volume, with maximum reduction of about 25% typically seen within 6 to 12 months of starting treatment.
Symptom Relief: By shrinking the prostate and relieving pressure on the urethra, finasteride improves lower urinary tract symptoms like weak urinary flow, frequency, and urgency.
Long-term Benefits: The medication reduces the long-term risk of severe BPH complications, including acute urinary retention and the need for surgery.
Affects PSA Levels: Finasteride lowers prostate-specific antigen (PSA) levels, requiring a baseline measurement and dose-adjusted interpretation during prostate cancer screening.
Potential Side Effects: Common side effects include erectile dysfunction, decreased libido, and reduced ejaculate volume, with a rare but controversial link to high-grade prostate cancer.
Ongoing Treatment: For the benefits to be maintained, finasteride treatment must be continuous; the prostate will begin to grow again if the medication is stopped.

Understanding the Mechanism: How Finasteride Works

Finasteride belongs to a class of drugs known as 5-alpha reductase inhibitors (5-ARI). Its primary function is to block the action of the 5-alpha reductase enzyme. This enzyme is responsible for converting testosterone into a more potent androgen, dihydrotestosterone (DHT). DHT plays a key role in the growth and enlargement of the prostate gland in men with benign prostatic hyperplasia (BPH). By inhibiting 5-alpha reductase, finasteride significantly lowers the levels of DHT in the prostate tissue and the bloodstream.

The reduction in DHT starves the prostate tissue of the primary growth stimulus, leading to a gradual decrease in the gland's size. This anti-androgen effect directly targets the underlying cause of BPH-related prostate enlargement, offering a disease-modifying approach to treatment rather than just symptom management.

Evidence of Prostate Size Reduction

Clinical studies have provided substantial evidence that finasteride effectively shrinks the prostate. The maximum reduction in prostate volume is typically achieved within the first 6 to 12 months of treatment.

Key Study Findings

MTOPS Trial: An analysis of data from the Medical Therapy of Prostatic Symptoms (MTOPS) trial showed that long-term treatment with 5 mg finasteride led to a consistent reduction of approximately 25% in total prostate volume compared to placebo. This effect was observed across the full range of baseline prostate sizes, including those considered small, moderate, and enlarged.
Long-term follow-up: Another study following patients for six years found that finasteride reduced prostate size by 20-25% within six months, and this reduction was sustained over the long term.
Benefits for larger prostates: Research indicates that men with larger prostates (typically over 30 cm³) at baseline experience the most significant benefits from finasteride, including greater symptom improvement and risk reduction.

Benefits of Prostate Shrinkage

The reduction in prostate size achieved with finasteride has several clinical benefits for men with BPH. By alleviating the pressure the enlarged prostate places on the urethra, it can lead to a marked improvement in urinary symptoms.

Primary benefits include:

Improved urinary flow: Reducing the physical obstruction allows for a stronger, more consistent stream.
Symptom relief: Patients report a decrease in bothersome lower urinary tract symptoms, such as urinary frequency, urgency, hesitancy, and the need to urinate at night (nocturia).
Reduced risk of complications: Long-term treatment with finasteride can decrease the likelihood of acute urinary retention (sudden inability to urinate) and the need for BPH-related surgery, such as transurethral resection of the prostate (TURP).

Comparing Finasteride to Other BPH Treatments

Finasteride is one of several treatment options for BPH, and its specific effects differ from those of other drug classes like alpha-blockers. Often, finasteride and alpha-blockers are used in combination to provide both rapid symptom relief and long-term prostate shrinkage.

Finasteride vs. Alpha-Blockers for BPH


Feature	Finasteride (e.g., Proscar)	Alpha-Blockers (e.g., Tamsulosin, Flomax)
Mechanism	Inhibits the enzyme 5-alpha reductase, reducing DHT levels and shrinking the prostate gland over time.	Relaxes the muscles in the prostate and bladder neck, which rapidly improves urine flow without changing prostate size.
Onset of Action	Slower, with a maximum effect on prostate size and symptoms typically taking 6 to 12 months.	Fast-acting, providing rapid improvement in urinary symptoms, often within days or weeks.
Effect on Prostate Size	Causes a clinically significant reduction in prostate volume (approximately 25%).	Does not reduce prostate size.
Primary Benefit	Long-term control of BPH progression and prevention of complications.	Rapid relief of bothersome urinary symptoms.
Side Effects	Primarily sexual side effects, including erectile dysfunction, decreased libido, and reduced semen volume.	More likely to cause dizziness, headaches, and low blood pressure.
Ideal Candidate	Men with significantly enlarged prostates or those seeking to prevent BPH progression.	Men with immediate symptomatic relief needs, often combined with finasteride for long-term management.

Important Considerations and Side Effects

While finasteride is a powerful tool for managing BPH, patients should be aware of potential side effects and other considerations.

Effects on PSA Levels

Finasteride affects prostate-specific antigen (PSA) levels, a biomarker used for prostate cancer screening. Because the medication shrinks the prostate, it reduces serum PSA concentrations by approximately 50% after about six months of treatment. A baseline PSA measurement should be taken before starting therapy. Doctors must account for this reduction when interpreting future PSA results, often by doubling the measured value for comparison to normal ranges.

Risk of High-Grade Prostate Cancer

There is some controversy regarding finasteride's effect on prostate cancer risk. The Prostate Cancer Prevention Trial (PCPT) found a reduced overall risk of prostate cancer but a slightly increased risk of high-grade prostate cancer in the finasteride group. However, experts believe this may be due to detection bias—finasteride's shrinking effect could make high-grade cancers easier to detect on biopsy. Patients should discuss this risk and the benefits of finasteride with their doctor.

Sexual Side Effects

Finasteride's mechanism of action, which involves suppressing androgens, can lead to sexual side effects. These can include:

Decreased libido (sex drive)
Erectile dysfunction (impotence)
Decreased volume of ejaculate

Some men have reported these sexual side effects, and in rare cases, they have persisted after discontinuing the medication, a condition sometimes referred to as 'Post-Finasteride Syndrome'.

Other Potential Side Effects

Other reported adverse effects are generally less common and may include:

Breast tenderness or enlargement (gynecomastia)
Allergic reactions, such as hives or swelling
Mood changes, including depression
Infertility

Conclusion

Does finasteride shrink the prostate? The answer is unequivocally yes. As a 5-alpha reductase inhibitor, finasteride actively reduces prostate size by lowering DHT levels, providing effective, long-term relief for symptoms of benign prostatic hyperplasia. This action not only improves urinary flow and comfort but also reduces the risk of long-term complications like urinary retention and the need for surgery. While generally well-tolerated, potential side effects, especially sexual ones, and the impact on PSA levels require careful consideration and discussion with a healthcare provider. Ultimately, finasteride remains a vital and effective medication for managing BPH, particularly for men with significantly enlarged prostates.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional before making any decisions about your health or treatment. Information regarding finasteride's effects on prostate size is based on scientific research, such as the analysis of the MTOPS trial data published in the Journal of Urology.

Frequently Asked Questions

It can take up to six months of consistent use to see the full effect of finasteride on prostate size and symptoms, although some improvements may be noticed sooner.

While finasteride works across a range of prostate sizes, its effects are most pronounced and beneficial for men with significantly enlarged prostates (typically over 30 cm³).

If you stop taking finasteride, your prostate will begin to grow again, and your BPH symptoms will likely return within a few months.

No, finasteride works slowly by shrinking the prostate. For faster symptom relief, particularly with weak stream and urgency, a doctor may initially prescribe an alpha-blocker, sometimes in combination with finasteride.

Finasteride reduces serum PSA levels by about 50% after six months of use. It is crucial to inform your doctor you are taking finasteride so they can adjust the interpretation of your PSA results for prostate cancer screening.

Studies have shown that finasteride can reduce the overall incidence of prostate cancer but is associated with a slightly higher risk of high-grade prostate cancer, possibly due to a detection bias. This complex relationship should be discussed with a healthcare provider.

Women who are pregnant or may become pregnant should not handle crushed or broken finasteride tablets due to the risk of absorption through the skin, which can cause abnormalities in a male fetus.