Finasteride and dutasteride are both 5-alpha reductase inhibitors that shrink the prostate by blocking DHT production — but they differ in mechanism breadth, potency, and time to full effect. The choice between them is rarely obvious and depends on prostate size, severity of symptoms, and tolerance for side effects.
Given the Modest Difference, Does It Really Matter Which One? Short Answer
For most men with moderate BPH and an average-sized prostate, the honest answer is that the clinical difference between the two is genuinely small — the 0.5-1 point IPSS gap found in head-to-head comparisons is unlikely to be something an individual man would notice as a meaningful difference in daily symptoms. Where the choice starts to matter more is at the extremes: men with a significantly enlarged prostate (over roughly 40-50cc on ultrasound measurement) tend to see more benefit from dutasteride's more complete DHT suppression, since a larger gland has more tissue mass for the drug to act on and a stronger DHT-suppression effect may translate to a more noticeable volume reduction in this population. Cost also becomes a bigger factor over time than the clinical evidence alone would suggest, given finasteride's dramatically lower price as a long-established generic versus dutasteride's higher cost even in generic form — for a medication typically taken indefinitely, this cost difference compounds significantly over years of use. For an average-risk man with a moderately enlarged prostate and no specific reason to prefer one over the other, starting with the cheaper, more extensively studied finasteride and reserving dutasteride for cases where finasteride alone proves insufficient is a reasonable, commonly used default.
How 5-ARIs Shrink the Prostate
DHT (dihydrotestosterone) — not testosterone — drives prostate cell growth. DHT is produced from testosterone by the enzyme 5-alpha reductase. Blocking this enzyme reduces intraprostatic DHT by 70–90%, causing prostate cells to shrink. The prostate volume reduction: 20–25% over 12 months. This reduces urethral compression and meaningfully lowers IPSS scores. Unlike alpha blockers, the benefit builds slowly but persists long-term.
Finasteride vs. Dutasteride: Mechanism Differences
- •Finasteride: inhibits only Type 2 5-alpha reductase — found primarily in the prostate and genital skin
- •Dutasteride: inhibits both Type 1 AND Type 2 5-alpha reductase — Type 1 is found in skin, liver, and scalp
- •Serum DHT suppression: finasteride 70%, dutasteride 90%
- •Intraprostatic DHT suppression: finasteride 85–90%, dutasteride 94–97%
- •Clinical consequence: despite greater DHT suppression, dutasteride shows only modest clinical superiority over finasteride in head-to-head trials
Efficacy: Which Reduces Prostate More?
Head-to-head comparisons show broadly similar clinical outcomes, although dutasteride suppresses DHT more completely and may give slightly greater prostate volume reduction without clearly better symptom relief. However, the CombAT trial and meta-analyses suggest the clinical difference in symptom scores between the two drugs is modest — approximately 0.5–1 point on the IPSS — and arguably not clinically meaningful for most men.
Side Effects: Sexual, Mood, and Long-Term
Both drugs carry similar sexual side effects. From clinical trials: reduced libido (3–6%), erectile dysfunction (3–5%), ejaculatory dysfunction (0.9–2.1%). Important: these trials may undercount side effects — post-marketing data and patient-reported outcomes suggest higher real-world rates. Mood-related side effects (depression, anxiety) are reported anecdotally and in some case series; regulatory agencies in some countries have added warnings.
Post-Finasteride Syndrome: Real Risk or Myth?
Post-finasteride syndrome (PFS) refers to persistent sexual, neurological, and psychological side effects that continue after stopping finasteride. PFS is documented in the medical literature — the FDA updated finasteride's label in 2012 to include persistent sexual side effects. Whether PFS represents a distinct syndrome or continuation of baseline symptoms remains debated. Men with pre-existing sexual dysfunction, anxiety, or mood disorders may be at higher risk.
Cost and Generic Availability
Generic finasteride 5mg: $5–15/month. Generic dutasteride 0.5mg: $30–60/month. Both are available as generics. The cost difference is significant over years of therapy — relevant when clinical efficacy difference is modest.
Switching Between the Two Medications
Men who start on finasteride and find symptom control inadequate after a genuine trial period (typically 6-12 months, since prostate volume reduction and symptom improvement build gradually) are the clearest candidates for switching to dutasteride, and this transition doesn't require a washout period — a man can generally switch directly from one 5-ARI to the other under medical guidance. The key consideration when switching is the half-life difference: finasteride clears from the body in days, while dutasteride's roughly 5-week half-life means any side effects experienced on dutasteride will persist considerably longer after discontinuation than the equivalent finasteride side effect would. This asymmetry is worth factoring into the decision — a man switching from finasteride to dutasteride is trading potentially better prostate-shrinking efficacy for a much longer commitment if he later wants to reverse course, which matters more for some men's risk tolerance than others.
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Frequently Asked Questions
Finasteride vs dutasteride which is better?
Dutasteride suppresses DHT more completely and may reduce prostate volume slightly more [1]. However, clinical symptom improvement (IPSS) is marginally different. Finasteride is cheaper, has more long-term safety data, and is typically first-line. Dutasteride is preferred for very large prostates or finasteride failures.
Dutasteride side effects
Same class as finasteride: reduced libido (4–6%), ED (3–5%), ejaculatory dysfunction. Gynecomastia (breast tenderness) is more commonly reported with dutasteride than finasteride. Persistent side effects after stopping are possible but less studied than with finasteride.
Does finasteride shrink the prostate?
Yes — finasteride gradually reduces prostate volume by blocking DHT-driven prostate cell growth. This reduction reduces urethral compression and improves IPSS scores, though the effect takes 3–6 months to become clinically apparent.
Is combination therapy with an alpha-blocker better than a 5-ARI alone?
For men with a larger prostate and moderate-severe symptoms, yes — the CombAT trial specifically showed combination therapy (dutasteride plus tamsulosin) outperformed either drug alone at reducing symptom progression and the risk of acute urinary retention or need for surgery over 4 years [2]. Combination therapy carries the combined side-effect burden of both drug classes, so it's generally reserved for men with more significant BPH rather than used as a universal starting point.
How is prostate size actually measured to decide between finasteride and dutasteride?
Prostate volume is typically measured via transrectal ultrasound or, increasingly, MRI when performed for other reasons, giving a volume in cubic centimeters (cc) or grams. A urologist uses this measurement alongside symptom severity (IPSS score) to help guide the choice between the two medications, with larger glands generally favoring dutasteride's more complete DHT suppression.
Does either drug reduce the need for BPH surgery down the road?
Yes — long-term data from trials like the CombAT study specifically shows that 5-ARIs (both finasteride and dutasteride) reduce the risk of acute urinary retention and the eventual need for BPH surgery over years of use, particularly in men with larger prostates, which is a meaningful long-term benefit beyond the day-to-day symptom improvement that's often the initial reason for starting treatment [2].
Is PSA still a reliable cancer screening tool while on either medication?
Yes, but only with the adjustment applied — both drugs reduce PSA by roughly 50%, so any PSA value while on either medication should be doubled for accurate comparison against standard reference ranges. Failing to account for this adjustment can cause a clinically significant PSA rise to be missed, which is why it's essential to inform any doctor ordering PSA testing that you're taking a 5-ARI.
Does prostate size shrink permanently, or does BPH return if the medication is stopped?
The volume reduction is not permanent — DHT levels rebound once the medication is stopped, and the prostate typically resumes its underlying growth trajectory, meaning symptoms and volume gains generally return over subsequent months. Both medications are intended for long-term, ongoing use rather than a fixed-course treatment, which is part of the cost consideration discussed above.
Can both drugs be used for hair loss as well as BPH in the same man?
Yes — some men are prescribed a 5-ARI for BPH who also have androgenetic alopecia benefit from the same underlying DHT-suppression mechanism addressing both conditions simultaneously with a single medication, though the doses used for BPH (finasteride 5mg, dutasteride 0.5mg) differ from the doses typically used for hair loss alone (finasteride 1mg), so this should be discussed explicitly with a prescriber.
Is there a maximum age at which starting a 5-ARI is no longer worthwhile?
No fixed age cutoff exists — the decision depends more on overall health, life expectancy, and symptom burden than age alone, though for a man with limited life expectancy from other health conditions, the gradual months-long benefit timeline of a 5-ARI may matter less than for a healthier man expecting many more years of benefit.
Do generic versions of both drugs work identically to brand-name Proscar and Avodart?
Yes — both generic finasteride and generic dutasteride meet the same FDA bioequivalence standards as their branded counterparts, with no expected efficacy difference, making generics the standard practical choice given the substantial cost savings, particularly relevant for medications typically taken long-term.