What is Finasteride?
Finasteride is a synthetic 4-azasteroid compound that specifically inhibits Type II 5α-reductase, the intracellular enzyme that converts testosterone to dihydrotestosterone (DHT), and is FDA-approved for treating benign prostatic hyperplasia (BPH) at 5 mg daily and male pattern hair loss (androgenetic alopecia) at 1 mg daily. 1
Mechanism of Action
Finasteride competitively and specifically inhibits Type II 5α-reductase, forming a stable enzyme complex with extremely slow turnover (half-life ~30 days), which prevents the conversion of testosterone to DHT in target tissues including the prostate, scalp, liver, and skin. 1
The drug has no affinity for the androgen receptor itself, but rather blocks DHT production—the potent androgen responsible for prostate enlargement and male pattern baldness. 1
A single 5 mg oral dose produces rapid reduction in serum DHT concentration, with maximum effect at 8 hours, and daily dosing at 5 mg reduces serum DHT by approximately 70% while increasing circulating testosterone by 10-20% (remaining within physiologic range). 1
In prostatic tissue specifically, finasteride reduces DHT content by approximately 80% compared to baseline, while testosterone tissue concentration increases up to 10-fold. 1
Clinical Indications and Dosing
For Benign Prostatic Hyperplasia (BPH)
The American Urological Association recommends finasteride 5 mg daily for men with lower urinary tract symptoms (LUTS) who have demonstrable prostatic enlargement (prostate volume >30cc), as it reduces prostate size by 15-25% within 6 months, improves peak urinary flow rate, and provides an average 3-point improvement in the AUA Symptom Index. 2, 3
Finasteride is ineffective and not appropriate for men with LUTS who do not have prostatic enlargement. 2, 3
The medication reduces the risk of acute urinary retention and need for BPH-related surgery, with absolute benefit increasing with rising prostate volume or serum PSA. 2
Symptom improvement is maintained for 6-10 years with continued therapy, though onset of action is slower than alpha-blockers (requiring 3-6 months for noticeable improvement). 2, 3
For Male Pattern Hair Loss (Androgenetic Alopecia)
Finasteride is FDA-approved at 1 mg daily for treatment of male pattern hair loss in men. 1, 4, 5
Clinical trials demonstrate visible hair growth in up to 66% of men with mild to moderate alopecia, and importantly stops hair loss in 91% of patients. 6
The drug increases the number and improves the shape of hair follicles in the anagen (growth) phase while decreasing DHT at the scalp level. 7
Pharmacokinetics
Finasteride is well absorbed after oral administration with a terminal elimination half-life of 4.7-7.1 hours, though slow accumulation occurs with multiple doses due to the drug's high affinity for the 5α-reductase enzyme. 8
The presence of food may slow absorption rate but does not affect total bioavailability. 8
The drug undergoes extensive hepatic metabolism to essentially inactive metabolites eliminated through bile and urine. 8
No dosage adjustments are necessary for elderly patients or those with renal dysfunction, though the effect of hepatic dysfunction is unknown. 8
Important Clinical Considerations
PSA Monitoring
Finasteride reduces serum PSA levels by approximately 50% after 1 year of therapy; when screening for prostate cancer in men taking finasteride, the measured PSA value should be doubled after 1 year to accurately assess disease progression. 2, 3, 9
The ratio of free/total PSA remains constant despite the reduction in absolute values. 2
Adverse Effects
Sexual side effects are the most common adverse reactions, including decreased libido (6.4% in first year, 2.6% in years 2-4), ejaculatory dysfunction (3.7% in first year, 1.5% in years 2-4), and erectile dysfunction. 2, 3
These side effects are typically reversible and become less common after the first year of therapy. 2
Gynecomastia occurs in approximately 4.5% of patients on long-term therapy. 2
Prostate Cancer Considerations
The American Society of Clinical Oncology/American Urological Association guidelines note that asymptomatic men with PSA ≤3.0 who undergo regular PSA screening may benefit from discussion about using 5α-reductase inhibitors for 7 years for prostate cancer prevention, though there is concern about a possible increase in high-grade prostate cancer. 2
The Prostate Cancer Prevention Trial (PCPT) demonstrated a 25% reduction in overall prostate cancer period prevalence with finasteride, though higher-grade tumors were more common in the finasteride arm. 2
Common Pitfalls to Avoid
Using finasteride in patients without prostatic enlargement is ineffective and exposes patients to unnecessary side effects. 2, 3
Failing to counsel patients about the slower onset of action (3-6 months) compared to alpha-blockers leads to premature discontinuation. 2, 3
Not adjusting PSA interpretation by doubling the measured value after 1 year of therapy can lead to missed prostate cancer diagnoses. 2, 3, 9
Assuming that finasteride alone provides optimal management for all BPH patients, when combination therapy with an alpha-blocker may be superior for those with larger prostates (>40cc) or more severe symptoms. 2, 9