How Does Finasteride Work? Mechanism of Action Explained in Plain English

Finasteride is often described simply as "a medication that blocks DHT." But what does that actually mean, and why does blocking DHT help with both hair loss and an enlarged prostate? This guide explains finasteride's mechanism of action in straightforward terms — no medical degree required.

What Is DHT and Why Does It Matter?

DHT stands for dihydrotestosterone. It's a sex hormone derived from testosterone — essentially a more potent version of testosterone that your body makes for specific purposes. DHT plays important roles in male development, but in adult men it's responsible for two unwanted effects:

Hair follicle miniaturization: In men genetically susceptible to androgenetic alopecia, DHT binds to receptors in scalp hair follicles and causes them to gradually shrink. Hair becomes finer, shorter, and eventually stops growing — the classic pattern of male pattern baldness.

Prostate growth: DHT is the primary driver of prostate enlargement. As DHT levels stimulate prostate cells to grow, the prostate enlarges and begins pressing on the urethra, causing urinary symptoms like frequent urination, weak stream, and difficulty starting urination.

The Enzyme That Makes DHT: 5-Alpha-Reductase

Testosterone doesn't turn into DHT on its own. It requires an enzyme called 5-alpha-reductase to catalyze the conversion. There are actually three types (isoenzymes) of this enzyme in the body:

Type I: Found mainly in the skin and liver

Type II: Dominant in the prostate, hair follicles, and liver — the primary isoenzyme responsible for prostate DHT production

Type III: Found in multiple tissues; less well studied

What Does Finasteride Actually Do?

Finasteride is a competitive inhibitor of 5-alpha-reductase types II and III. In plain terms, finasteride molecules bind to the enzyme and block it from converting testosterone into DHT — like putting a key in a lock to prevent the right key from working.

The result: significantly lower DHT levels in the blood and tissues. Studies show that:

Finasteride 1 mg reduces serum DHT levels by approximately 65–70% within 24 hours of the first dose

Finasteride 5 mg reduces serum DHT by approximately 70–75%

Scalp DHT (the type responsible for hair follicle miniaturization) is also significantly reduced

How This Treats Hair Loss

With DHT levels dramatically reduced in the scalp, hair follicles are no longer under the same hormonal pressure to miniaturize. Over months, finasteride:

Slows the progression of hair loss in nearly all men with androgenetic alopecia

Allows previously miniaturized follicles to partially recover and produce thicker, longer hair

Increases total hair count over a 12-month period in most men

However, finasteride is not a cure — it only works as long as you keep taking it. When discontinued, DHT levels return to normal, and hair loss resumes. Most newly regrown hair is lost within 12 months of stopping.

How This Treats BPH

In the prostate, lowering DHT removes the primary hormonal driver of prostate cell growth. Over 6–12 months, finasteride:

Reduces prostate volume by an average of 25–28% from baseline

Improves urinary flow rate and reduces symptom severity

Reduces the risk of acute urinary retention (sudden inability to urinate) by about 57% in long-term studies

Reduces the risk of needing surgery (TURP or prostatectomy) by about 55% in long-term studies

Why Does Finasteride Affect PSA Test Results?

PSA (prostate-specific antigen) is produced by prostate cells. As finasteride shrinks the prostate, PSA output falls. Finasteride reduces serum PSA by approximately 50% within 6 months. This is not dangerous in itself — but it means any PSA results taken while on finasteride must be interpreted with this reduction in mind. A PSA that looks "normal" could actually be elevated relative to what it should be post-finasteride. Always tell your doctor you're taking finasteride whenever prostate cancer screening is discussed.

What About Finasteride and Testosterone Levels?

Because finasteride blocks the conversion of testosterone to DHT, testosterone levels in the blood actually increase slightly. Finasteride does not lower testosterone. This is a common misconception. The drug specifically reduces DHT, not total testosterone, which is why it doesn't cause the muscle loss or energy effects associated with testosterone deficiency.

For a full discussion of side effects, see: Finasteride Side Effects: What to Expect and When to Call Your Doctor.

For a full overview of finasteride's uses and dosing, see: What Is Finasteride? Uses, Dosage, and What You Need to Know in 2026.