Dutasteride

Avodart

Dutasteride (Avodart) is a hormonal & sexual research compound. Inhibits BOTH type 1 and type 2 5-alpha-reductase, suppressing serum DHT by over 90% versus finasteride's ~70%.

How Dutasteride works

Inhibits BOTH type 1 and type 2 5-alpha-reductase, suppressing serum DHT by over 90% versus finasteride's ~70%.

Proposed benefits

Researched for libido, hormonal signaling and reproductive / sexual function.

Where to get Dutasteride

The evidence for Dutasteride

Graded by what exists behind each claim.

✅ Clinically validated

• Approved for BPH with large randomized data, and trialed for hair loss where it outperformed finasteride in a head-to-head study.

📊 Correlative data

• Used for more aggressive hair-loss control. Reported side effects mirror finasteride's and are generally described as more likely — consistent with the broader enzyme blockade and the very long half-life.

🧪 Theoretical / extrapolated

• Inhibits both type 1 and type 2 5-alpha-reductase, suppressing serum DHT by ~90% against finasteride's ~70%.
• The half-life is the practical difference nobody plans for: 4–5 weeks, so it takes months to clear after stopping. That predicts both the stronger effect and the slower recovery if someone reacts badly.

How to read these tiers: they say how much human evidence exists, not how well something works — and ✗ flags harm, never a disappointing trial. How the evidence tiers work →

What Dutasteride actually does

There are two 5-alpha-reductases, they are different genes in different tissues, and the entire difference between dutasteride and finasteride is which ones they hit. Both isoenzymes do the same chemistry: NADPH-dependent reduction of the C4–C5 double bond in ring A of testosterone, giving 5α-dihydrotestosterone, a ligand that binds the androgen receptor several times more tightly than testosterone does and dissociates more slowly.

Type 2 (SRD5A2) is the prostate, genital skin and hair follicle enzyme, and it is the one finasteride was built for — the label calls finasteride a type 2 inhibitor and dutasteride an inhibitor of both type 1 and type 2 Nickel 2004 US Food and Drug Administration 2020. Type 1 (SRD5A1) sits in sebaceous glands, liver and non-genital skin, and it keeps making DHT in a person on finasteride. That surviving isoenzyme is the reason a type 2 inhibitor cannot drive serum DHT to the floor, and it is why the two drugs are not the same drug at different doses.

The size of the difference, measured. Dutasteride 0.5 mg daily reduced serum DHT by a mean of 90.2% and a median of 93.7% (p < 0.001) in 4,325 men Roehrborn 2002; the label reports median reductions of 94% at 1 year, 93% at 2 years and 95% at 3 and 4 years, with 85–90% reached after a single week US Food and Drug Administration 2020. Nickel 2004's comparison of the two drug programs states the result plainly: dutasteride reduces serum DHT significantly more than finasteride does, while producing similar prostate volume reduction, similar flow-rate and symptom improvement and similar reductions in long-term progression risk. That sentence is the whole clinical bargain in one line — more enzyme blockade, the same benefit on the prostate.

Where the extra blockade does buy something is hair, and there is a systematic review. Almudimeegh 2024 pooled nine studies — four randomized controlled trials, one single-arm, two prospective and two retrospective cohorts — and found dutasteride at 0.5 mg and 2.5 mg significantly more effective than finasteride 1 mg for hair count, with no significant difference in adverse events between them. Mechanistically that fits: the scalp carries both isoenzymes, so blocking only type 2 leaves local DHT production running.

One structural detail that explains the half-life. Both drugs are 4-azasteroids and both are mechanism-based inhibitors: the enzyme starts to reduce them and ends up covalently trapped in a complex with NADP+. The dissociation of that complex is extraordinarily slow, and dutasteride's is slower. That is the molecular origin of the number that dominates every practical decision on this page — a terminal half-life of about five weeks US Food and Drug Administration 2020, against finasteride's 4.7 to 7.1 hours Steiner 1996. Same class, same chemistry, a difference of roughly two orders of magnitude in how long you are committed.

Cell, rodent, human — and where it stops

Step one, enzyme and dose-ranging in humans. Gisleskog 1999 gave single oral doses from 0.01 to 40 mg to 32 healthy men and modeled the result. The output is unusual and it matters: dutasteride has parallel linear and nonlinear elimination. Volume of distribution 511 L, linear clearance 0.58 L/h, a saturable route with a Km of 0.96 ng/mL and a maximum saturable clearance of 6.2 L/h. Below about 0.1 mg daily the nonlinear pathway dominates; above 1 mg the linear one does. In plain terms, the enzyme itself is part of the clearance mechanism, and once you saturate it the drug behaves completely differently.

Step two, the BPH program, which met its endpoints. Roehrborn 2002 pooled three trials: 4,325 men enrolled, 2,951 completing 24 months on 0.5 mg daily or placebo. Prostate volume fell 25.7% and transition zone volume 20.4% (p < 0.001). Symptom score improved 4.5 points (21.4%) and peak flow rose 2.2 mL/s at 24 months, with flow significant from month 1 and symptoms from month 6. Acute urinary retention risk fell 57% and BPH surgery 48%.

Step three, REDUCE — and this is the trial that has to be read completely. Andriole 2010 randomized 6,729 men aged 50–75 with a PSA of 2.5–10.0 ng/mL and one previous negative biopsy to dutasteride 0.5 mg or placebo for four years, with protocol biopsies. Prostate cancer was found in 659 of 3,305 on dutasteride against 858 of 3,424 on placebo — a 22.8% relative risk reduction (95% CI 15.2–29.8, p < 0.001). That is the headline and it is real.

Now the rest of it. Gleason 7–10 tumors over years 1–4 were 220 versus 233 — p = 0.81, no difference at all. So the entire cancer reduction was in low-grade disease, much of which would never have mattered. And in years 3 and 4 there were 12 Gleason 8–10 tumors on dutasteride against 1 on placebo (p = 0.003). The label carries that as a warning in its own right: 1.0% versus 0.5% Gleason 8–10 in men aged 50 to 75 US Food and Drug Administration 2020. A third signal is easy to miss: cardiac failure in 30 men (0.7%) on dutasteride against 16 (0.4%) on placebo, p = 0.03.

Step four, what the drug does to the test used to find that cancer. Andriole 2011 analyzed PSA performance inside REDUCE and found something counter-intuitive: once PSA is correctly halved and interpreted, it performs better on dutasteride, not worse. The area under the ROC curve for detecting Gleason 7–10 disease was 0.700 on dutasteride versus 0.650 on placebo (p = 0.0491) for final PSA, and 0.699 versus 0.593 (p = 0.0001) for the change in PSA from month 6. Suppressing the benign contribution to PSA sharpens the signal from the malignant one — provided somebody actually does the doubling.

The obstacle, and it is a screening obstacle rather than a biological one. Everything above was measured in men aged 50 to 75 with an elevated PSA and a prior biopsy, taking the drug for the prostate. The men taking it on this site are frequently in their twenties and thirties, taking it for hair, for years, and are outside every prostate cancer screening pathway that would catch the REDUCE signal. The high-grade finding was detected because 6,729 men were being biopsied on protocol. Nobody is biopsying a 28-year-old on dutasteride for hair loss, and the long-term hair-loss population has never been followed for a prostate endpoint at all.

Dutasteride pharmacokinetics — how much of it actually gets in

Route: oral once daily, and this is the longest-committed compound in the vault. Absolute bioavailability is approximately 60% (range 40–94%) with peak concentration at 2 to 3 hours; food reduces peak concentrations by 10–15%, which the label calls clinically insignificant US Food and Drug Administration 2020.

The half-life is the whole risk calculus. Terminal elimination half-life is approximately 5 weeks at steady state, and serum concentrations reach 65% of steady state after 1 month and about 90% after 3 months US Food and Drug Administration 2020. Gisleskog 1999 measured a range of 1 to 7 weeks at high concentrations and about 3 days at low ones — the saturable clearance route again. Work the arithmetic in the direction that matters: five half-lives is 25 weeks. Somebody who reacts badly at week six and stops that day is still carrying measurable drug in June if they stopped in January. Finasteride's 4.7–7.1 hour half-life Steiner 1996 means the same decision costs a few days. That asymmetry, not potency, is the argument for trying the weaker drug first.

What degrades it. Extensive hepatic metabolism by CYP3A4 and CYP3A5 to hydroxylated metabolites US Food and Drug Administration 2020. Excretion is mainly in feces — about 5% as unchanged drug and 40% as dutasteride-related metabolites — with only trace amounts in urine. A strong CYP3A4 inhibitor (ritonavir, ketoconazole, verapamil, diltiazem) raises exposure of a drug that already accumulates for three months, and the label does not give a dose adjustment for it.

Dose-frequency reasoning, done honestly. Because the half-life is roughly 35 days, dosing every second or third day changes the steady-state concentration by far less than the dosing interval suggests — the plasma level barely moves between doses. That is the pharmacokinetic basis for the 0.5 mg two-to-three-times-weekly pattern this site records, and it is sound. What it does not do is make the compound easier to stop; the accumulation and the washout are unchanged. There is no injectable dutasteride, and there is no topical formulation with published human pharmacokinetics, so the oral capsule and its five-week tail are the only exposure profile that exists.

What would have to be true, and how you would know it was not

Four predictions. The fourth is the one that argues against the compound, and it is the reason the third one is not optional.

1. DHT should collapse, and it should do so within a week. Draw Total & Free DHT at baseline and at 4 weeks. The expectation is a fall of roughly 90% Roehrborn 2002, with 85–90% already present after seven days US Food and Drug Administration 2020. A fall of only 60–70% is the finasteride-shaped answer and suggests either a dosing gap or a product that is not what it says it is. Draw Total Testosterone in the same tube: with the conversion route blocked, testosterone typically drifts up modestly and stays inside the reference range.

2. PSA should halve, and the number you write down is not the number the lab prints. The label is explicit: dutasteride reduces serum PSA by approximately 50% within 3 to 6 months, and after three months on treatment the PSA value should be doubled for comparison with normal ranges US Food and Drug Administration 2020. Draw a baseline before starting — there is no way to reconstruct it afterwards — and again at 6 months. The free-to-total ratio stays constant, so percent-free PSA is still interpretable at face value.

3. A rising PSA on this drug is more informative than a rising PSA off it. This is the prediction most people have backwards. Andriole 2011 found that PSA change discriminated Gleason 7–10 disease with an ROC area of 0.699 on dutasteride versus 0.593 on placebo. Any confirmed rise from the on-treatment nadir should be treated as a real signal rather than noise, because the benign contribution to the number has been removed. Symptom scores belong alongside it: IPSS and uroflow at baseline and 6 months, where the expectation from Roehrborn 2002 is about 4.5 points and 2.2 mL/s.

4. The prediction that cuts against it: the cancers this drug prevents are the ones that were least likely to hurt you, and the high-grade count moved the wrong way. In REDUCE the 22.8% reduction was entirely in low-grade disease; Gleason 7–10 was 220 versus 233, p = 0.81, and in years 3–4 Gleason 8–10 was 12 versus 1, p = 0.003 Andriole 2010. So the falsifiable claim is this: if dutasteride were genuinely chemopreventive rather than merely shrinking the benign tissue that hides tumors, high-grade counts should have fallen too. They did not. Anyone taking this for prostate protection is taking it on a result the trial did not produce.

What nobody has tested yet

Nobody has followed young men taking dutasteride for hair loss for a prostate endpoint. REDUCE detected its high-grade signal because every participant was biopsied on protocol Andriole 2010. The population that now takes this drug for decades starting in its twenties has no such surveillance and no registry. A prospective cohort of long-term hair-loss users with annual PSA — correctly doubled — and a defined biopsy trigger would either reproduce the signal in the population that matters or retire it. Twenty-five years after approval this does not exist.

Nobody has measured scalp DHT on dutasteride versus finasteride. The entire mechanistic argument for dual inhibition in hair is that the follicle carries type 1 as well as type 2, and Almudimeegh 2024 shows the clinical superiority without ever measuring the tissue. Scalp biopsies with tissue DHT quantified by mass spectrometry, in men randomized to each drug, would show whether the extra hair comes from the extra local enzyme blockade or from something else — and would settle whether a topical dual inhibitor could work at all.

Nobody has explained the cardiac failure signal. REDUCE reported cardiac failure in 0.7% versus 0.4% (p = 0.03) Andriole 2010, an imbalance with no proposed mechanism and no follow-up trial. It could be chance; it is also the kind of finding that only resolves with a dedicated cardiovascular analysis in a 5-alpha-reductase inhibitor cohort, which nobody has published.

And nobody has run a withdrawal study. With a five-week half-life and 25 weeks to full washout, the natural question for anyone who develops sexual or mood side effects is what the recovery curve looks like. Serial DHT, testosterone and validated sexual function scores every four weeks for six months after stopping, in thirty men, would produce the first real answer to the most common question asked about this class.

Dutasteride — its own safety story, not its class's

The PSA consequence is the most consequential fact on this page, and it is a screening problem rather than a side effect. Dutasteride halves PSA within 3 to 6 months, and the label instructs that after three months the value must be doubled before it is compared with a normal range US Food and Drug Administration 2020. The failure mode is concrete: a 32-year-old starts dutasteride for hair, never records a baseline, and twenty years later presents with a PSA of 3.0 that everybody reads as reassuring when the untreated equivalent is 6.0. Get a baseline PSA before the first capsule, keep it, and tell any future clinician you are on this drug. That single sentence is worth more than the rest of this section.

The high-grade prostate cancer warning, in the label's own numbers. In men aged 50 to 75 in REDUCE there was an increased incidence of Gleason 8–10 prostate cancer — 1.0% on dutasteride versus 0.5% on placebo US Food and Drug Administration 2020 — concentrated in years 3 and 4, where the counts were 12 versus 1 Andriole 2010. Whether that is detection bias from a shrunken gland or a real biological effect has never been resolved, and it belongs on the page either way.

Sexual and breast adverse events, at the rates the trials measured. In the 24-month monotherapy trials, months 0–6: impotence 4.7% versus 1.7%, decreased libido 3.0% versus 1.4%, ejaculation disorders 1.4% versus 0.5%, breast disorders 0.5% versus 0.2% US Food and Drug Administration 2020. Those are placebo-controlled excesses of a few percent, and they are front-loaded in the first six months. The five-week half-life is what makes them serious: a problem that appears at month two takes months to leave.

The neuropsychiatric signal, reported honestly in both directions. Garcia-Argibay 2022 followed 2,236,876 Swedish men, of whom 8,774 took dutasteride and 70,645 finasteride. Adjusted hazard ratios on dutasteride were 1.10 (1.01–1.20) for all-cause dementia, 1.28 (1.09–1.50) for Alzheimer disease, 1.31 (1.08–1.59) for vascular dementia and 1.68 (1.43–1.96) for depression, with no association with suicide (0.98, 0.62–1.54). The authors note the dementia association diminished over time while the depression association did not. This is an observational cohort in men whose median age at treatment was 73, so confounding by indication is a live objection — but a 68% relative increase in depression across two million men is not a finding to leave off the page.

Two hard rules from the label that have nothing to do with the user. Men on dutasteride must not donate blood until at least 6 months after the last dose, because a pregnant transfusion recipient would receive the drug. And the capsules should not be handled by a woman who is or may be pregnant, because dutasteride is absorbed through skin and poses a risk to a developing male fetus US Food and Drug Administration 2020. The six-month blood donation window is a direct readout of the five-week half-life, and it is the most under-observed instruction on this label.

Sources read for this page

• Roehrborn CG, Boyle P, Nickel JC. Efficacy and safety of a dual inhibitor of 5-alpha-reductase types 1 and 2 (dutasteride) in men with benign prostatic hyperplasia.. Urology 2002 · PMID 12350480
• Andriole GL, Bostwick DG, Brawley OW. Effect of dutasteride on the risk of prostate cancer.. N Engl J Med 2010 · PMID 20357281
• Andriole GL, Bostwick D, Brawley OW. The effect of dutasteride on the usefulness of prostate specific antigen for the diagnosis of high grade and clinically relevant prostate cancer in men with a previous negative biopsy: results from the REDUCE study.. J Urol 2011 · PMID 21074214
• Gisleskog PO, Hermann D, Hammarlund-Udenaes M. The pharmacokinetic modelling of GI198745 (dutasteride), a compound with parallel linear and nonlinear elimination.. Br J Clin Pharmacol 1999 · PMID 10073740
• Nickel JC. Comparison of clinical trials with finasteride and dutasteride.. Rev Urol 2004 · PMID 16985923
• Steiner JF. Clinical pharmacokinetics and pharmacodynamics of finasteride.. Clin Pharmacokinet 1996 · PMID 8846625
• Almudimeegh A. Comparison between dutasteride and finasteride in hair regrowth and reversal of miniaturization in male and female androgenetic alopecia: a systematic review.. Dermatol Reports 2024 · PMID 39749123
• Garcia-Argibay M, Hiyoshi A, Fall K. Association of 5alpha-Reductase Inhibitors With Dementia, Depression, and Suicide.. JAMA Netw Open 2022 · PMID 36547981
• US Food and Drug Administration. AVODART (dutasteride) soft gelatin capsules, for oral use - full prescribing information.. FDA approved labeling, revision 2020

Dutasteride — safety, predicted from mechanism

Dutasteride — interference & stacking

Bloodwork to run alongside Dutasteride

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Dutasteride — frequently asked questions

Dutasteride inside a finished plan

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What Dutasteride is used for

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