Finasteride

• Benign Prostatic Hyperplasia (BPH)
• Male Pattern Hair Loss
• Prostate Enlargement
• Androgenetic Alopecia

Finasteride (Proscar 5 mg, Propecia 1 mg) is a 5-alpha reductase inhibitor used to treat benign prostatic hyperplasia and male-pattern hair loss. By suppressing dihydrotestosterone (DHT) — the androgen primarily responsible for prostate growth and follicular miniaturization — it shrinks the prostate gland over months and can stabilize or partially reverse hereditary balding. The two FDA-approved doses (1 mg for hair loss, 5 mg for BPH) are not interchangeable and serve very different patient populations, although the underlying drug and mechanism are identical.

Mechanism of Action

Finasteride is a competitive, selective inhibitor of type II 5-alpha reductase, the enzyme isoform that converts testosterone to dihydrotestosterone in the prostate, hair follicles, and scalp skin. DHT is roughly five times more potent than testosterone at the androgen receptor, so reducing tissue DHT by approximately 70% has dramatic effects on androgen-driven processes without lowering circulating testosterone or causing systemic hypogonadism — an important distinction from agents like LHRH agonists used in advanced prostate cancer, which suppress testosterone broadly and produce castration-level androgen deprivation with all the systemic consequences that implies.

In the prostate, sustained DHT suppression leads to apoptosis of glandular epithelial cells and a 20–30% reduction in prostate volume over 6–12 months. In the scalp, restoring follicles toward normal-caliber growth requires similar timelines because hair grows in cycles and the visible benefit lags the biochemical effect by several months. Unlike dutasteride, which inhibits both type I and type II isoenzymes and lowers DHT by approximately 90%, finasteride spares the type I enzyme found in skin sebaceous glands and liver. This narrower inhibition profile may translate into a slightly different side-effect profile, although dutasteride and finasteride show comparable efficacy in BPH at a population level. Some clinicians choose dutasteride when finasteride has produced incomplete response, on the theory that more comprehensive enzyme inhibition might rescue refractory cases.

The drug has a relatively short plasma half-life (5–6 hours in younger men, longer in elderly), but its biological effect outlasts plasma exposure because of irreversible enzyme inhibition. This pharmacodynamic feature allows once-daily dosing despite shorter plasma kinetics.

Clinical Use

For BPH with moderate-to-severe lower urinary tract symptoms and a measurably enlarged prostate (generally >40 mL or PSA >1.5 ng/mL as a surrogate for size), finasteride 5 mg daily is well-supported by the MTOPS and PLESS trials, which demonstrated reduced symptom progression, lower rates of acute urinary retention, and decreased need for prostate surgery over 4–6 years of follow-up. Effect onset is slow — meaningful symptom improvement may not be apparent for 6 months and full benefit can take a year. For more rapid symptom relief, an alpha-blocker such as tamsulosin, alfuzosin, silodosin, terazosin, or doxazosin is often added; combination therapy outperforms either drug alone in men with both enlarged prostates and bothersome symptoms. The AAFP and AUA both endorse this combined approach for symptom-dominant disease with prostate enlargement. For overactive bladder symptoms that coexist with BPH, anticholinergics like oxybutynin, solifenacin, tolterodine, fesoterodine, or darifenacin, or the beta-3 agonists mirabegron and vibegron, can be added with monitoring for urinary retention.

For androgenetic alopecia, finasteride 1 mg daily is FDA-approved for men with vertex and anterior mid-scalp hair loss. Frontotemporal recession is less reliably reversed because the receding frontal hairline often reflects more advanced miniaturization. Topical minoxidil is often used concurrently, and the combination outperforms either alone. Hair benefits require continuous use; discontinuation results in loss of regrown hair within 12 months as DHT-driven miniaturization resumes. Some patients use lower doses (less than 1 mg daily) with reasonable cosmetic outcomes — though this is off-label, the underlying enzyme kinetics support it.

How to Take It

Finasteride may be taken with or without food at any consistent time of day. Pregnant women — and women who could become pregnant — must not handle crushed or broken tablets, because the drug can be absorbed through the skin and cause hypospadias in a male fetus. Intact tablets are coated and safe to handle. Store at room temperature away from moisture.

Men on finasteride should not donate blood until at least one month after their last dose, to avoid exposure of pregnant transfusion recipients. Patients should be counseled that hair regrowth (or BPH symptom relief) develops slowly over months, not weeks, and that stopping the drug reverses gains over the following year. Set realistic expectations about cosmetic hair benefit: most men maintain their existing hair and see modest regrowth in the crown; a complete cosmetic restoration is uncommon and patients hoping for hairline reconstruction will be disappointed.

Set realistic expectations regarding sexual side effects as well: 2–4% of men experience decreased libido, erectile dysfunction, or reduced ejaculatory volume during treatment. Most cases resolve with continued use or after discontinuation, but post-finasteride syndrome — persistent symptoms after stopping — has been reported in postmarketing data and remains a topic of active study. Discuss this risk before initiation rather than after a complaint emerges. PDE-5 inhibitors such as sildenafil and tadalafil can be used safely with finasteride if erectile dysfunction develops; tadalafil also has FDA approval for BPH symptoms and is sometimes used as monotherapy or in addition.

For patients struggling with the timing or adherence of multiple men's-health medications, our men's health checklist reviews comprehensive routine care.

Monitoring and Follow-Up

Finasteride lowers serum PSA by approximately 50% within 6–12 months. To screen for prostate cancer in men on this drug, the measured PSA value should be doubled for comparison to age-adjusted reference ranges. Any rise in PSA from a stable on-treatment baseline warrants urologic evaluation, even when the absolute number remains within "normal" limits — finasteride may mask but does not prevent prostate cancer, and a rising trend on therapy is more concerning than a similar trend off therapy. Annual digital rectal examination remains appropriate. The understanding blood work article explains how PSA is interpreted in context, and our review of cancer screenings by age outlines age-appropriate cancer screening more broadly.

For BPH, an objective symptom score (AUA Symptom Index or IPSS) at baseline and at 6-month intervals helps quantify response. A post-void residual measurement and uroflow study are sometimes useful when symptoms are not improving. For hair loss, photographic documentation under standardized lighting at 6 and 12 months is more useful than subjective impressions. Periodic LFTs are not routinely required but reasonable in patients with hepatic impairment or polypharmacy. Renal function does not require routine monitoring specifically for finasteride.

Special Populations

Finasteride is contraindicated in women of childbearing potential and is not approved for women or children. Postmenopausal women have been studied for hair loss with negative or mixed results, although some specialists use it off-label in selected cases. Elderly men do not require dose adjustment, although polypharmacy and changes in renal and hepatic function with age can affect tolerance of this and any concurrent medications. No adjustment is needed for renal impairment. In hepatic impairment, finasteride should be used cautiously — there are no specific dose recommendations but drug clearance is reduced.

The Prostate Cancer Prevention Trial (PCPT) suggested a small absolute increase in high-grade (Gleason 8–10) prostate cancers among finasteride users, although total prostate cancer incidence was reduced by approximately 25%. Whether this represents a true biological effect or a detection artifact (smaller prostates yielding higher-grade tumors more readily on biopsy) remains debated. Discuss this nuanced risk profile with appropriate patients, particularly those weighing chemoprevention. For men with significant cardiovascular comorbidity such as coronary artery disease or heart failure, finasteride does not pose specific cardiac risk.

When to Contact Your Doctor

Report: a sudden inability to urinate (acute retention requiring catheterization); new breast pain, lumps, or nipple discharge (rare male breast cancer has been reported); persistent depressed mood or thoughts of self-harm; severe persistent sexual dysfunction; or any rash or facial swelling suggesting allergy. Authoritative drug information is available at MedlinePlus and the FDA prescribing label.

If you have questions about finasteride or your treatment plan, our team at Zimmer Medical Group can help — contact us or schedule a visit.