5-Alpha-Reductase Inhibitors
Finasteride and Dutasteride - BPH, Hair, and the Sexual Trade-offs
Evidence grade 5 of 5, Settled. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- FDA Approved (Finasteride 5mg 1992, 1mg 1997; Dutasteride 2001)
Reference only. Not medical advice.
What can go wrong
Sexual adverse effects (decreased libido, ED, reduced ejaculate, occasionally gynecomastia) are the main concern. 5-ARIs roughly halve PSA, so values must be doubled for cancer screening interpretation. They are teratogenic - women who are or may become pregnant must not handle crushed/broken tablets. A debated high-grade prostate cancer signal appears in FDA labeling. Mood changes have been reported.
This is something to take to a clinician. The Atlas can tell you what is known about 5-alpha-reductase inhibitors in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
Finasteride and dutasteride inhibit 5-alpha-reductase, lowering dihydrotestosterone (DHT). At higher doses they shrink the prostate for BPH; at lower doses (finasteride 1mg) they slow androgenetic hair loss. The same DHT suppression that delivers these benefits underlies documented sexual adverse effects - decreased libido, erectile dysfunction, and reduced ejaculate volume - reported across a wide range in trials, with real-world data suggesting these effects are notably more frequent with finasteride than dutasteride.
They shrink the prostate and regrow hair by cutting DHT - and that is exactly where the sexual side effects come from.
What people take it for
What it is taken for. Approval, where it exists, covers only some of these.
How it works
5-alpha-reductase converts testosterone to DHT, the dominant androgen in the prostate and hair follicle. Finasteride preferentially inhibits the type II isoenzyme; dutasteride inhibits both type I and type II, producing deeper DHT suppression. Lower DHT shrinks prostatic tissue over 6-12 months and prolongs the hair follicle growth phase. Because DHT also supports libido, erectile signaling, and ejaculatory tissue, its suppression can produce sexual effects.
The DHT trade-off and the question of persistence
Marketing frames these drugs as cosmetic or symptomatic with mild, reversible side effects. The harder reality: ejaculation-disorder, ED, and low-libido signals are dose- and drug-dependent (substantially higher with finasteride than dutasteride in real-world data), and a contested entity - post-finasteride syndrome - describes sexual, neurological, and mood symptoms some men report as persisting after discontinuation. Causation and frequency of persistent symptoms remain debated, which is precisely why honest counseling matters.
Safety in detail
These are prescription drugs with documented sexual, endocrine, and reproductive effects requiring physician evaluation and PSA-interpretation adjustments. Self-sourcing for hair loss without monitoring carries documented risks.
What has to be monitored
The bloodwork this needs, and the reason it belongs with a clinician rather than a website.
- PSA (interpret as doubled while on 5-ARI)
- Sexual function / libido at follow-up
- Mood changes
- Breast changes / gynecomastia
What people get wrong
Often saidSide effects are trivial and always reversible
ActuallySexual side effects are real, dose-related, and more common with finasteride; a subset of men report persistent symptoms (post-finasteride syndrome), an entity whose frequency and causation remain debated.
Often said5-ARIs lower testosterone
ActuallyThey lower DHT, not testosterone - circulating testosterone typically rises modestly as conversion is blocked.
Who profits, and who pushes back
Nobody is named here yet, because we have not done that work on this entry. Read that as unfinished. Somebody almost certainly sells this, and somebody almost certainly disputes it. Assume both until we have checked.
Set against the alternatives
Versus alpha-blockers, 5-ARIs work slowly (months) but actually shrink the gland and reduce long-term progression, retention, and surgery risk; alpha-blockers act within days but do not change prostate size. Dutasteride suppresses DHT more completely than finasteride and, in real-world data, carries a lower reported sexual side-effect burden.
At a glance
- Mechanism
- Blocks 5-alpha-reductase, lowers DHT
- Finasteride isoenzyme
- Type II (and III)
- Dutasteride
- Dual type I + II inhibitor
- Hair dose
- Finasteride 1mg/day
- BPH dose
- Finasteride 5mg/day
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about 5-Alpha-Reductase Inhibitors. A short entry is not a safe entry.
- Timeline. We have not built the dated record of research and regulatory events.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- Children and teenagers. We have not checked what is known about younger people.
- Where we land. We have not taken a position on this yet.
Sources
The studies below are named and described, but not linked. You cannot open them from here and check our reading against them, which means this page is our summary of the literature rather than a document you can verify. Weigh it accordingly until the links exist.
The studies that matter
Comprehensive review of 5-ARI sexual side effects2010s
Sexual adverse effects reported across trials roughly 2% to 38%; most common are ED, ejaculatory dysfunction, decreased libido
Real-world finasteride vs dutasteride comparison2020s
Ejaculation-disorder risk reported several-fold higher with finasteride; ED and low-libido risk also higher vs dutasteride
5-ARI and high-grade cancer signal (PCPT/REDUCE context)2000s-2010s
5-ARIs lowered overall prostate cancer detection but raised a debated signal for high-grade tumors, informing FDA labeling