DHT, 5-Alpha-Reductase & Finasteride
Dihydrotestosterone, the Conversion Enzyme, and the Drugs That Block It
Evidence grade 5 of 5, Settled. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- Finasteride FDA Approved (1992 BPH, 1997 hair loss)
Reference only. Not medical advice.
What can go wrong
Sexual side effects (reduced libido, erectile and ejaculatory dysfunction) in a minority; contested persistent post-finasteride symptoms; mood effects reported; finasteride halves PSA, complicating prostate cancer screening if unadjusted; both agents are teratogenic and must not be handled by or given to pregnant women.
This is something to take to a clinician. The Atlas can tell you what is known about dht, 5-alpha-reductase & finasteride 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.
Dihydrotestosterone (DHT) is the more potent androgen, produced from testosterone by the enzyme 5-alpha-reductase. DHT binds the androgen receptor more tightly than testosterone and drives prostate growth, scalp hair-follicle miniaturization (male pattern baldness), and much of male external genital development. 5-alpha-reductase inhibitors, finasteride (type 2 selective) and dutasteride (types 1 and 2), block this conversion, lowering DHT by roughly 70% (finasteride) to over 90% (dutasteride). They treat benign prostatic hyperplasia and androgenetic alopecia, and finasteride at 1 mg is the basis of Propecia.
The more potent androgen most men have never heard of, and the pill that suppresses it.
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 irreversibly converts testosterone to DHT, which binds the androgen receptor with higher affinity and slower dissociation than testosterone, amplifying androgenic signaling in target tissues (prostate, scalp, skin, genitals). Finasteride competitively inhibits the type 2 isoenzyme; dutasteride inhibits both type 1 and type 2, producing deeper DHT suppression.
Blocking DHT does not lower your testosterone, and that is the catch
5-alpha-reductase inhibitors lower DHT while testosterone itself stays the same or rises slightly. So they do not cause 'low testosterone' in the lab sense. But DHT mediates real functions, and suppressing it can cause sexual side effects, decreased libido, erectile difficulty, reduced ejaculate, in a minority of users. A subset report persistent symptoms after stopping ('post-finasteride syndrome'), a contested and incompletely understood entity. On the TRT side, the DHT pathway explains why testosterone therapy can accelerate scalp hair loss and oily skin/acne: more substrate feeds more DHT in susceptible men. Finasteride also lowers PSA by roughly half, which must be accounted for in prostate cancer screening.
Safety in detail
5-alpha-reductase inhibitors alter androgen metabolism and PSA interpretation and can cause sexual side effects. Use requires evaluation by a qualified physician; women who are or may become pregnant must not handle these drugs due to teratogenic risk.
What it does to fertility
5-alpha-reductase inhibitors can reduce ejaculate volume and have been associated with reversible changes in semen parameters in some men; they are distinct from testosterone in that they do not suppress the HPG axis, but fertility concerns warrant discussion. They are teratogenic to a male fetus.
What has to be monitored
The bloodwork this needs, and the reason it belongs with a clinician rather than a website.
- Baseline and PSA adjustment (multiply measured PSA by ~2 after 6-12 months on therapy)
- Sexual function and mood assessment
- Avoidance of handling by pregnant women
What people get wrong
Often saidFinasteride lowers your testosterone
ActuallyFinasteride lowers DHT, not testosterone, total testosterone is unchanged or slightly higher; the side effects come from reduced DHT signaling, not low testosterone.
Often saidTRT-related hair loss means the testosterone dose is too high
ActuallyHair loss reflects DHT conversion in genetically susceptible follicles, not simply dose, which is why a 5-alpha-reductase inhibitor, not dose reduction alone, is the targeted intervention.
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.
At a glance
- Enzyme
- 5-alpha-reductase (types 1 & 2)
- Finasteride DHT drop
- ~70%
- Dutasteride DHT drop
- > 90%
- Finasteride approvals
- BPH 1992, hair loss 1997
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about DHT, 5-Alpha-Reductase & Finasteride. 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.
- How it compares. We have not set it against the alternatives.
- 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
PLESS / MTOPS (finasteride in BPH)1998-2003
Established finasteride efficacy in BPH and prostate volume reduction
Prostate Cancer Prevention Trial (PCPT, finasteride)2003
Reduced overall prostate cancer incidence; clarified PSA adjustment