5-Alpha-Reductase Inhibitors: Shrinking the Gland
An alpha-blocker relaxes the prostate in a day, but it never makes the gland any smaller — it just widens the channel through an obstruction that keeps growing. 5-alpha-reductase inhibitors take the opposite, slower road: they starve the prostate of the hormone that feeds it, and over months the gland physically shrinks. They are the only oral drugs for benign prostatic hyperplasia that change the disease rather than merely easing its symptoms — and the price of reaching into the androgen pathway is a set of systemic effects, and a laboratory trap that can hide a prostate cancer if you don't know it's there.
A 68-year-old man has been getting up four times a night. His stream is weak and hesitant, and he feels he never fully empties. On examination his prostate is smoothly enlarged — a big gland, not a hard, craggy one. He started tamsulosin a month ago and his flow is a little better, but the urologist wants to add a second drug — not for tonight's symptoms, but for the next five years. His prostate is large enough that, left alone, it carries a real risk of acute urinary retention and eventual surgery. The second drug will not help him this week. But taken every day for months, it will slowly shrink the gland itself — treating not the muscle tone around the obstruction, but the obstruction. Before he leaves, the urologist notes one thing in the chart: from now on, double whatever PSA the lab reports.
Two problems in one gland
Obstruction in BPH has a dynamic part and a static part — and they need different drugs. The narrowing that blocks urine flow in benign prostatic hyperplasia comes from two sources. The dynamic component is smooth-muscle tone in the prostate and bladder neck, squeezing the channel shut; this is the target of alpha-blockers such as tamsulosin, which relax that muscle within days. The static component is the sheer bulk of glandular tissue — the physical mass of an enlarged prostate pressing on the urethra. No amount of muscle relaxation removes bulk. To treat the static component you have to make the gland itself smaller, and that is exactly what 5-alpha-reductase inhibitors do. The two drug classes attack the two components, which is why they are so often used together — the split between dynamic and static obstruction is developed fully in the Alpha-blocker and Combination-therapy chapters.
The hormone that grows the prostate
The prostate is an androgen-dependent organ: it grows and maintains itself in response to male sex hormones. But the androgen that actually drives prostatic growth is not testosterone itself. Inside the prostate, the enzyme 5-alpha-reductase converts testosterone into dihydrotestosterone (DHT), a far more potent androgen — it binds the androgen receptor several times more avidly. DHT is the local signal that tells prostatic tissue to proliferate. Block the enzyme and you starve the gland of its own growth signal: prostatic DHT falls, the tissue is no longer told to grow, and over months existing bulk regresses. Note the elegance — you are not blocking testosterone throughout the body, only the conversion step that concentrates androgen action inside the prostate. The androgen pathway itself, and the testosterone-to-DHT conversion, are taught in full in the Endocrine section; here we apply it.
The drugs: finasteride and dutasteride
Both block the enzyme; they differ in how many of its isoforms they hit. 5-alpha-reductase exists as more than one isoenzyme. Finasteride selectively inhibits the type-2 isoenzyme, which is the dominant form in the prostate. Dutasteride inhibits both type-1 and type-2 isoenzymes, so it suppresses DHT more completely — prostatic DHT falls further and serum DHT falls further too. In practice the two drugs produce broadly similar clinical benefit in BPH; dutasteride's dual blockade gives a more thorough hormonal suppression rather than a dramatically different outcome. Either way, the effect is not fast. Because the gland shrinks by regression rather than relaxation, meaningful symptom improvement typically takes three to six months, and the reduction in the risk of retention and surgery accrues over years of continued use.
Think of the obstructed urethra as a garden hose squeezed inside a swelling fist. The alpha-blocker relaxes the fist so the hose opens up — instant, but the fist can clench again. The 5-alpha-reductase inhibitor does something slower and more fundamental: it turns off the water supply that keeps the fist swelling, so over weeks the fist itself gets smaller. One eases the grip today; the other shrinks the hand that grips.
The single highest-yield fact about these drugs is what they do to PSA. By shrinking prostatic tissue, a 5-alpha-reductase inhibitor lowers serum prostate-specific antigen (PSA) by roughly 50% within six to twelve months. If you interpret that halved value as if it were normal, you can be falsely reassured and miss a prostate cancer. So in a man taking finasteride or dutasteride, you must DOUBLE the measured PSA before comparing it to reference ranges — and a PSA that rises despite the drug is a red flag that demands investigation. This dovetails with the Genitourinary cancers / prostate chapter, where PSA interpretation is the core skill.
Finasteride (Proscar) — inhibits 5-alpha-reductase type 2; 5 mg daily for BPH. Dutasteride (Avodart) — inhibits types 1 and 2; 0.5 mg daily. Combination product: dutasteride plus tamsulosin (Combodart / Jalyn) in a single capsule, pairing static shrinkage with dynamic relaxation. And the same enzyme, a different dose, a different disease: finasteride 1 mg (Propecia) and dutasteride are used for male-pattern hair loss — because scalp hair follicles are also DHT-driven.
- BPH obstruction = dynamic (muscle tone, alpha-blockers) + static (glandular bulk, 5-ARIs).
- 5-alpha-reductase converts testosterone to DHT, the potent androgen that drives prostate growth.
- Finasteride blocks type 2; dutasteride blocks types 1 and 2 for fuller DHT suppression.
- Onset is slow — 3 to 6 months for symptom benefit; the gland shrinks rather than relaxes.
- Disease-modifying: lowers the risk of acute retention and BPH-related surgery, best in larger glands.
- They lower serum PSA by ~50% — always double the value when screening for cancer.
Slow, but the only drug that changes the disease
Because relief is slow, a 5-alpha-reductase inhibitor is a poor choice as sole therapy for a man who wants his nocturia fixed now — and this is where the two classes complement each other. A common strategy is to start an alpha-blocker for prompt symptom control and add a 5-alpha-reductase inhibitor for the long game, particularly when the prostate is large and the goal is to prevent progression. Long-term trials showed that combination therapy reduced the risk of clinical progression, acute urinary retention, and the need for surgery more than either drug alone, with the greatest benefit in men with larger, higher-risk glands. In smaller prostates the shrinkage has little to work with, so a 5-alpha-reductase inhibitor is reserved for demonstrably enlarged glands. Some men are eventually stepped off the alpha-blocker once the gland has shrunk and symptoms are stable.
The systemic price: sex, breasts, and pregnancy
Reach into the androgen pathway and the effects reach beyond the prostate. Because they lower a potent androgen, 5-alpha-reductase inhibitors carry a characteristic set of hormonal adverse effects. The commonest are sexual: reduced libido, erectile dysfunction, and ejaculatory problems (including reduced ejaculate volume). Gynaecomastia — breast tenderness or enlargement — can occur because suppressing androgen shifts the oestrogen-to-androgen balance. A minority of men report that sexual and mood symptoms persist after stopping the drug (sometimes called post-finasteride syndrome), and regulators have added warnings about a possible signal of depression and, rarely, suicidal ideation. None of this is usually dangerous, but all of it must be counselled before starting, because a man expecting a harmless prostate pill can be blindsided by loss of libido.
The most absolute safety point concerns pregnancy, not the man taking the drug. Because DHT is essential for the normal development of a male fetus's external genitalia, a 5-alpha-reductase inhibitor is teratogenic. A woman who is or may become pregnant must not handle crushed or broken tablets — the drug can be absorbed through the skin — and finasteride and dutasteride are absorbed into semen, so condom use is advised to avoid exposing a pregnant partner. Intact, film-coated tablets are safe to handle; the hazard is broken tablets and semen exposure. This is a hard contraindication to memorise, in the same category as the other classic teratogens.
Same enzyme, other uses — and a cancer debate
Because scalp hair follicles and prostate share the same DHT-driven biology, low-dose finasteride and dutasteride are mainstays for androgenetic alopecia (male-pattern baldness), and are sometimes used for hirsutism — the crossover taught alongside the antiandrogens in the Dermatology chapter. Finally, a genuine controversy every student should be able to state. Large prevention trials found that finasteride and dutasteride reduced the overall incidence of prostate cancer, yet showed a slightly higher rate of high-grade cancers in the treated groups. Much of that signal is now thought to reflect detection artefact — a shrunken gland makes biopsy sampling more sensitive to aggressive tumours — rather than the drugs causing worse cancer. Regulators nonetheless note the finding, and it belongs in any honest counselling discussion. The nuance is developed in the Genitourinary cancers / prostate chapter.
- Sexual adverse effects are commonest: reduced libido, erectile dysfunction, ejaculatory problems.
- Gynaecomastia can occur; some men report persistent effects and a possible depression signal — counsel first.
- Teratogenic: pregnant/potentially-pregnant women must not handle crushed tablets; drug is in semen.
- Same drugs, lower dose: finasteride/dutasteride for androgenetic alopecia (DHT-driven follicles).
- Prevention trials: lower overall prostate cancer but a debated high-grade cancer signal (likely detection bias).
- Reading a PSA at face value in a man on finasteride/dutasteride. The drug halves PSA — double it, or you may miss a cancer.
- Starting a 5-alpha-reductase inhibitor alone for a man who wants fast relief. It takes months — add an alpha-blocker for early symptom control.
- Letting a pregnant partner handle broken tablets or ignoring semen exposure. The drug is teratogenic to a male fetus.
A 70-year-old man has taken dutasteride for a large prostate for a year. His PSA is reported as 2.4 ng/mL. How should this be interpreted when screening for prostate cancer?
- 5-alpha-reductase inhibitors (finasteride, dutasteride) shrink the prostate by blocking testosterone→DHT conversion, treating the static component of BPH obstruction.
- Onset is slow (3–6+ months) but disease-modifying — they cut the risk of acute retention and surgery, best in large glands, and pair well with an alpha-blocker for early relief.
- They lower serum PSA by ~50% — always double the value when screening for prostate cancer, or you may miss it.
- Systemic androgen effects: sexual dysfunction, gynaecomastia, a possible depression signal, and teratogenicity (women must not handle crushed tablets); same drugs treat androgenetic alopecia.
- NICE guideline NG97 — Lower urinary tract symptoms in men: management (BPH pharmacotherapy).
- EAU Guidelines on Management of Non-Neurogenic Male Lower Urinary Tract Symptoms, including Benign Prostatic Obstruction.
- Rang & Dale's Pharmacology — Drugs affecting the reproductive tract; androgens and antiandrogens.
- Katzung Basic & Clinical Pharmacology — Androgens & antiandrogens; drugs used in benign prostatic hyperplasia.
- British National Formulary (BNF) — Finasteride, dutasteride, and combination products.
- McConnell JD, et al. The Medical Therapy of Prostatic Symptoms (MTOPS) trial. New England Journal of Medicine; and Thompson IM, et al. Prostate Cancer Prevention Trial (PCPT).
