Finasteride reduces DHT levels by approximately 70% by blocking the 5-alpha reductase enzyme, which halts the miniaturization process that drives male pattern hair loss. This article explains exactly how this mechanism works at the biological level, why it is effective for androgenetic alopecia, and what its limitations are.
This content is for informational purposes only and does not constitute medical advice.
The Role of DHT in Hair Loss
Dihydrotestosterone (DHT) is an androgen hormone derived from testosterone. In men genetically predisposed to pattern baldness, DHT binds to androgen receptors on hair follicles in specific scalp regions, primarily the temples, crown, and mid-scalp.
What DHT Does to Follicles
When DHT binds to a susceptible follicle, it triggers a process called follicular miniaturization:
- Anagen phase shortens: The active growth phase decreases from 2-6 years to just weeks or months
- Hair shaft thins: Each growth cycle produces a finer, shorter, less pigmented hair
- Follicle shrinks: The follicle itself physically reduces in size
- Terminal to vellus conversion: Thick terminal hairs become thin, colorless vellus hairs
- Follicle death: After enough miniaturization cycles, the follicle can no longer produce any hair
This process does not affect all scalp hair equally. The follicles along the back and sides of the head (the donor area) typically lack the androgen receptors that make frontal and vertex follicles vulnerable to DHT.
How Finasteride Blocks DHT
Testosterone is converted to DHT by an enzyme called 5-alpha reductase. There are two types of this enzyme:
| Enzyme Type | Primary Location | Role |
|---|---|---|
| Type I 5-alpha reductase | Skin, liver, sebaceous glands | Minor contributor to scalp DHT |
| Type II 5-alpha reductase | Hair follicles, prostate | Primary driver of scalp DHT production |
Finasteride selectively inhibits Type II 5-alpha reductase. By blocking this enzyme, finasteride reduces serum DHT concentrations by approximately 70% and scalp DHT by a similar margin.
The Molecular Process
At the molecular level, finasteride acts as a competitive inhibitor. It binds to the active site of the Type II enzyme, preventing testosterone from occupying that site and being converted. The binding is tight and long-lasting, which is why a single daily 1 mg dose maintains consistently suppressed DHT levels throughout the day.
This differs from dutasteride (0.5 mg daily), which inhibits both Type I and Type II 5-alpha reductase and produces more complete DHT suppression. However, dutasteride is not FDA-approved for hair loss and carries a higher incidence of side effects.
What Happens at the Follicle Level
With DHT levels reduced, the miniaturization process slows or stops. The biological cascade works in reverse:
- Reduced DHT binding: Fewer DHT molecules reach androgen receptors on follicles
- Anagen phase extends: Follicles spend more time in active growth
- Hair shaft thickens: Vellus hairs may begin converting back to intermediate or terminal hairs
- Follicle recovery: Miniaturized but still-living follicles can resume normal function
This recovery is why 65% of men on finasteride experience visible regrowth, not just stabilization. The drug gives shrinking follicles the hormonal environment they need to start producing thicker hairs again.
Recovery Takes Time
Hair follicles cycle through growth (anagen), regression (catagen), and resting (telogen) phases. A single cycle takes months. For finasteride to produce visible results, follicles need to complete at least one full cycle under reduced DHT conditions.
| Phase | Duration | What Happens |
|---|---|---|
| Anagen (growth) | 2-6 years | Active hair production |
| Catagen (regression) | 2-3 weeks | Follicle detaches and shrinks |
| Telogen (resting) | 3-4 months | Hair falls out; follicle rests before new cycle |
This is why finasteride takes 3-6 months to show results and why the 12-month evaluation point is the clinical standard.
Where Finasteride Works Best by Norwood Stage
Finasteride is most effective when follicles are miniaturized but not permanently dead. The earlier you start, the more follicles can be rescued.
| Norwood Stage | Finasteride Effectiveness | Grafts If Transplant Needed |
|---|---|---|
| Stage 2 | Highly effective; most follicles recoverable | 800 to 1,500 |
| Stage 3 | Very effective; strong regrowth potential | 1,500 to 2,200 |
| Stage 3V | Effective; vertex responds well | 2,000 to 2,800 |
| Stage 4 | Moderately effective; stabilization likely, regrowth possible | 2,500 to 3,500 |
| Stage 5 | Limited regrowth; good for stabilizing remaining hair | 3,000 to 4,500 |
| Stage 6-7 | Stabilization only; dead follicles cannot recover | 4,000 to 7,500 |
To determine your current stage, use the free AI assessment at myhairline.ai/analyze. Knowing your Norwood stage helps you and your doctor set realistic expectations for what finasteride can achieve.
Limitations of the Mechanism
Finasteride cannot do the following:
- Revive dead follicles: Once a follicle has fully atrophied, no medication can bring it back. Only a hair transplant (FUE recovery is 7-10 days with 90-95% graft survival) can restore hair in those areas.
- Block all DHT: Finasteride reduces DHT by about 70%, not 100%. Some miniaturization may still occur, especially in highly sensitive individuals.
- Work without consistency: Missing doses allows DHT to rebound. The mechanism requires continuous enzyme inhibition.
- Treat non-androgenetic causes: Autoimmune hair loss, nutritional deficiency, thyroid conditions, and medication-induced shedding have different root causes that DHT reduction does not address.
For a detailed comparison of when medication is enough versus when surgery is needed, see our finasteride vs hair transplant guide.
Assess Your Hair Loss Stage
Understanding the mechanism helps you make informed decisions, but the first step is knowing where your hair loss currently stands. The free AI tool at myhairline.ai/analyze uses facial landmark detection to classify your Norwood stage in under 60 seconds. Your result gives you a baseline for tracking finasteride's effectiveness over time. Read our complete finasteride guide for dosing, side effects, and long-term protocols.
This article is for educational purposes only. It does not replace professional medical advice. Consult a board-certified dermatologist or hair restoration specialist before making treatment decisions.