What Is Finasteride?
Finasteride is a daily oral pill that blocks the enzyme responsible for converting testosterone into dihydrotestosterone (DHT). It's FDA-approved for male pattern baldness at 1 mg daily and prostate enlargement at 5 mg daily.
For men on testosterone replacement therapy, finasteride serves a specific purpose: preventing or slowing hair loss. Exogenous testosterone can accelerate genetic hair thinning in men predisposed to androgenetic alopecia. Finasteride reduces circulating DHT by approximately 70%, which halts the miniaturization of hair follicles in the scalp.1
Brand names include Propecia (1 mg) and Proscar (5 mg). Generic formulations and compounded versions are widely available. The drug requires daily administration and takes 3-6 months to show measurable effects on hair preservation.
Finasteride at a Glance
Finasteride is a daily oral pill that reduces DHT by 70%, preventing hair loss in men genetically predisposed to male pattern baldness.
It's commonly prescribed alongside TRT to counteract testosterone-driven hair thinning.
- Mechanism: Blocks 5-alpha reductase, stopping testosterone-to-DHT conversion
- Dosing: 1 mg daily by mouth, takes 3-6 months to show effect
- Safety: Sexual side effects in 2-4%, requires PSA monitoring
- Cost: $10-30/month generic, widely available via telehealth
How Finasteride Works
Finasteride blocks 5-alpha reductase, the enzyme that converts testosterone to DHT. It binds to the enzyme's active site with high affinity, physically preventing testosterone from accessing the conversion pathway.1 This competitive inhibition is selective: finasteride targets the type II isoform of 5-alpha reductase, which predominates in scalp hair follicles and prostate tissue, with 100-fold greater selectivity than the type I isoform found in skin.2
DHT reduction happens quickly. A single 1-mg dose suppresses serum DHT by approximately 65% within 24 hours.3 Steady-state suppression stabilizes within weeks of consistent dosing.
The Hair Loss Connection
Male pattern baldness occurs when DHT binds to androgen receptors in genetically susceptible hair follicles, triggering a cascade that shrinks follicles over time. Smaller follicles produce thinner, shorter hairs until they eventually stop producing visible hair altogether.
Finasteride interrupts this process by lowering DHT levels in the scalp. With less DHT available to bind follicle receptors, miniaturization slows or stops entirely. This preserves existing hair density and prevents further progression of androgenetic alopecia.1
Pharmacokinetics
Finasteride has a short half-life of 6-8 hours in men under 60, which is why daily dosing is required for sustained DHT suppression.4 The drug is well absorbed orally — food may delay absorption slightly but doesn't affect total bioavailability.5
Metabolism occurs primarily in the liver via the CYP3A4 enzyme system. About 90% of circulating finasteride binds to plasma proteins. The two main metabolites retain less than 20% of the parent compound's activity, meaning most of the clinical effect comes from unchanged finasteride.2
5-Alpha Reductase is an enzyme that converts testosterone into DHT (dihydrotestosterone), a more potent androgen involved in hair loss and prostate growth. Finasteride blocks this enzyme to reduce DHT production.
Competitive Inhibition occurs when a drug molecule binds to an enzyme's active site, physically blocking the enzyme's natural substrate from accessing and being converted. This prevents the normal biochemical reaction from occurring.
Dosing & Administration
Finasteride for hair loss is dosed at 1 mg once daily, taken orally with or without food. The 5-mg formulation is reserved for benign prostatic hyperplasia and is not appropriate for hair preservation purposes.
Start finasteride before beginning TRT if hair preservation is a priority, or add it concurrently with testosterone initiation. Most clinicians recommend baseline and periodic monitoring of PSA levels, since finasteride reduces PSA by approximately 50% — an important consideration when screening for prostate cancer.3
| Parameter | Standard Protocol | Clinical Notes |
|---|---|---|
| Starting Dose | 1 mg PO daily | Take at the same time each day. Food has no significant impact on absorption. |
| Titration | None required | 1 mg is the standard and maximum dose for hair loss. Higher doses do not improve efficacy. |
| Onset of Effect | 3-6 months | DHT suppression occurs within weeks, but visible hair preservation takes months. |
| Lab Monitoring | PSA at baseline and 6-12 months | Finasteride reduces PSA by ~50%. Clinicians should double the PSA value when interpreting results. |
| Testosterone Monitoring | If on TRT, standard TRT labs | Finasteride does not alter testosterone levels but blocks its conversion to DHT. |
If you miss a dose, take it as soon as you remember unless it's almost time for the next dose. Never double up. Consistency matters — intermittent dosing allows DHT levels to fluctuate, reducing efficacy.
Side Effects & Safety
Finasteride is generally well tolerated, but sexual side effects are the most commonly reported adverse events. Mood changes and libido reduction occur in a minority of users.
| Frequency | Side Effect | Clinical Context |
|---|---|---|
| Common (>10%) | None consistently reported | Most men tolerate 1-mg dosing without noticeable side effects. |
| Occasional (1-10%) | Decreased libido, erectile dysfunction, ejaculatory dysfunction | Sexual side effects occur in 2-4% of users. Most resolve with discontinuation. |
| Occasional (1-10%) | Mood changes, depressive symptoms | Neuropsychiatric effects are reported but difficult to separate from baseline TRT-related mood shifts. |
| Rare (<1%) | Gynecomastia, testicular pain | Breast tissue changes are uncommon but warrant evaluation if persistent. |
| Rare (<1%) | Post-finasteride syndrome (persistent side effects after discontinuation) | Controversial and poorly characterized. Incidence unclear but likely very low. |
Liver disease may impair finasteride metabolism, but dose adjustment is not routinely recommended. Men with significant hepatic impairment should discuss risks with their prescriber.
Cost & Access
Generic finasteride 1 mg costs $10-30 per month through most retail and mail-order pharmacies. Brand-name Propecia runs $70-90 monthly but offers no clinical advantage over generic equivalents.
Compounded finasteride from telehealth clinics typically falls in the $15-25 range and is often bundled with testosterone prescriptions. Some men save further by purchasing 5-mg Proscar tablets and splitting them into quarters (1.25 mg), though this requires a separate prescription and manual pill-cutting.
Insurance Coverage
Most insurance plans do not cover finasteride for hair loss, classifying it as a cosmetic indication. Coverage for the 5-mg dose (BPH treatment) is standard, which is why some men use the cost-splitting strategy above.
Finasteride does not require prior authorization when prescribed for androgenetic alopecia, since it's available over-the-counter in some formulations. Cash-pay pricing is competitive enough that most men skip insurance entirely.
Where to Get It
Finasteride is available through traditional urologists and dermatologists, primary care physicians, and telehealth TRT providers. Most online men's health platforms (Hims, Keeps, Roman) offer finasteride as a standalone service or as part of a bundled hair-loss protocol that includes minoxidil.
Compounding pharmacies can prepare custom finasteride formulations, including topical solutions that some men prefer to minimize systemic DHT suppression. Topical finasteride is not FDA-approved but is widely prescribed off-label.