5-Alpha reductase inhibitors are a major class of medications used to treat benign prostatic hyperplasia (BPH) and male pattern baldness (androgenic alopecia). The most commonly used drug in this class is Finasteride, known by the brand names Proscar (BPH dose) and Propecia (hair-loss dose).
Finasteride works by altering the hormonal pathway that enlarges the prostate and contributes to hair thinning. Because it affects testosterone metabolism, students must understand its therapeutic benefits as well as its hormonal adverse effects and contraindications.
What Are 5-Alpha Reductase Inhibitors?
5-Alpha reductase inhibitors are drugs that block the enzyme responsible for converting testosterone into dihydrotestosterone (DHT) — a stronger androgen that enlarges the prostate and contributes to hair loss.
Common medications include:
- Finasteride
- Dutasteride (another longer-acting option)
These medications are taken orally (PO) and are primarily used in men.
Classification
Pharmacologic Class (P): 5-Alpha Reductase Inhibitors
Therapeutic Class (T): BPH Drugs
Uses of Finasteride
Finasteride is used for two major clinical conditions:
Benign Prostatic Hyperplasia (BPH)
- Reduces prostate gland size
- Improves urinary flow
- Reduces nighttime urination
- Prevents acute urinary retention
It is especially effective in men with enlarged prostates.
Male Pattern Hair Loss (Androgenic Alopecia)
At a lower dose (1 mg/day), finasteride slows hair loss and promotes regrowth in many men.
Additional Uses (Less Common)
- Part of transgender hormone therapy (rare, but used for DHT suppression)
- Adjunct therapy in prostate cancer research (not first-line)
Mechanism of Action (MOA)
Finasteride works by inhibiting 5-alpha reductase, the enzyme that converts testosterone into dihydrotestosterone (DHT).
Why this matters:
DHT enlarges the prostate → leads to BPH symptoms
DHT causes hair follicle shrinkage → leads to hair thinning and lossBy lowering DHT levels, finasteride:
- Shrinks the prostate gland
- Reduces BPH symptoms
- Slows or reverses hair loss
This hormonal modification is the reason for both its therapeutic effects and side effects.
Adverse Effects of 5-Alpha Reductase Inhibitors
A helpful mnemonic from your chart is BPH (matching the disease it treats):
B – Breast Tissue Enlargement (Gynecomastia)
Due to altered androgen–estrogen balance.
P – Penile Dysfunction
Includes:
- Erectile dysfunction
- Decreased libido
- Ejaculation disorders
These side effects may persist even after stopping the medication in rare cases ("post-finasteride syndrome").
H – Hypotension
Occurs especially when combined with other BPH medications like alpha-blockers.
Additional Side Effects
- Depression or low mood
- Reduced semen volume
- Male infertility (rare and reversible)
- Rash
- Testicular discomfort
Because finasteride affects hormones, it can have sexual and psychological effects.
Drug Interactions
Finasteride has no major drug–drug interactions, which makes it safer than many other hormonal medications.
However:
Caution with alpha-blockers (tamsulosin, doxazosin)→ may increase risk of low blood pressure
Overall, interaction risk is low.
Contraindications
Finasteride should NOT be used in:
Pregnancy Category X
- Absolutely contraindicated
- Can cause severe fetal abnormalities, especially external genital malformations in male fetuses
- Pregnant women should not handle crushed or broken tablets due to risk of absorption through the skin
Prostate Cancer Risk
Men with suspected or known prostate cancer should avoid finasteride because:
- It lowers PSA levels (masking cancer detection)
- It may increase the risk of high-grade prostate cancer in rare cases
Other Contraindications
- Liver disease (use with caution)
- Children (not indicated)
Clinical Pearls for Students
- Finasteride lowers PSA levels by 50% → clinicians must adjust PSA interpretation.
- Takes 3–6 months to show full benefits in BPH.
- Takes 3–12 months to show improvement in hair growth.
- Works best in men with clearly enlarged prostates (>40 g).
- Most effective when combined with alpha-blockers for symptomatic relief.
- Should not be handled by pregnant women.
These are highly testable points in NCLEX and pharmacology exams.
Patient Education
Patients should be taught to:
- Expect delayed results (several months)
- Report breast enlargement or lumps
- Continue therapy long-term for sustained benefit
- Understand that sexual side effects may occur
- Keep tablets away from pregnant women
- Monitor mood changes or depressive symptoms
Proper counseling improves compliance and reduces anxiety related to hormonal side effects.
5-Alpha reductase inhibitors like finasteride are essential medications for managing BPH and male pattern baldness. By blocking the conversion of testosterone to DHT, they shrink the prostate and slow hair loss. Although generally safe, finasteride has hormonal side effects such as gynecomastia, sexual dysfunction, and an increased risk of hypotension. It is strictly contraindicated in pregnancy and requires PSA monitoring in men.
This class is frequently tested in exams, and understanding its mechanism, uses, and safety precautions is essential for future nurses, doctors, and pharmacists.
FAQs on 5-Alpha Reductase Inhibitors (Finasteride)
1. How long does finasteride take to work for BPH?
Usually 3–6 months for noticeable improvement.
2. Does finasteride treat urinary symptoms immediately?
No. It shrinks the prostate slowly. Alpha-blockers give faster relief.
3. Can women use finasteride?
Not recommended. Absolutely contraindicated in pregnancy.
4. Does finasteride cure baldness?
It slows hair loss and may promote regrowth but works only while taking the medication.
5. Can finasteride cause infertility?
Rarely. It may temporarily reduce semen volume or quality.
6. Why does finasteride affect sexual function?
Because lowering DHT changes androgen activity in sexual tissues.
7. Is finasteride safe long term?
Yes, when monitored properly, but PSA levels must be interpreted carefully.
