5-α Reductase Inhibitors Drug-Drug Interaction Table

What Is the 5-α Reductase Inhibitor Drug Class?

5-alpha reductase inhibitors reduce intracellular DHT by blocking the enzyme responsible for converting testosterone to its more potent androgen, dihydrotestosterone. Clinically available agents are finasteride and dutasteride. Both are approved for benign prostatic hyperplasia (BPH); only finasteride 1 mg holds FDA approval for male-pattern hair loss (androgenetic alopecia). The FDA drug label for finasteride (Proscar) and dutasteride (Avodart) define the approved uses and safety profiles.

Isoenzyme Selectivity: Why It Matters Clinically

The human genome encodes two primary 5-alpha reductase isoenzymes. Type I is expressed predominantly in the scalp and skin. Type II is the dominant isoenzyme in the prostate and genital tissue.

Finasteride selectively inhibits type II, producing a mean 65 to 70% reduction in serum DHT. Dutasteride inhibits both type I and type II, driving serum DHT down by up to 90 to 98% at steady state according to its FDA-approved label. That broader enzymatic blockade underpins dutasteride's more complete androgen suppression and its longer, more complex pharmacokinetic profile.

Pharmacokinetics at a Glance

| Parameter | Finasteride | Dutasteride | |---|---|---| | Bioavailability | ~65% | ~60% | | Protein binding | ~90% | ~99% (albumin + AAG) | | Primary CYP pathway | CYP3A4 (minor contributor) | CYP3A4 / CYP3A5 (major) | | Active metabolites | No clinically relevant ones | Mono- and di-hydroxydutasteride | | Half-life | ~6 h (up to 8 h in elderly) | ~3 to 5 weeks | | Elimination | Fecal (~57%), renal (~39%) | Fecal (~40%), minimal renal |

The 5-week half-life of dutasteride has direct DDI implications. Because the drug stays in systemic circulation for months after discontinuation, an interaction with a CYP3A4 inhibitor started several weeks after stopping dutasteride may still produce elevated drug levels.

Mechanism-Based Drug Interactions: Finasteride

Finasteride is metabolized by CYP3A4, but only as a minor pathway. A pharmacokinetic study published in the Journal of Clinical Pharmacology showed that ketoconazole 400 mg co-administration increased finasteride AUC by approximately 76% in healthy volunteers, a statistically significant but generally tolerated change given finasteride's wide therapeutic index. No dose adjustment is mandated by the FDA label, though monitoring is appropriate.

CYP3A4 Inhibitors with Finasteride

Strong CYP3A4 inhibitors that may raise finasteride exposure include:

• Azole antifungals: ketoconazole, itraconazole, voriconazole
• HIV protease inhibitors: ritonavir, cobicistat-boosted regimens
• Macrolide antibiotics: clarithromycin (not azithromycin)
• Others: grapefruit juice at large volumes (irrelevant at typical dietary quantities)

The clinical consequence of a 76% AUC increase is greater DHT suppression. For BPH patients, this is unlikely to cause harm. For a patient using finasteride 1 mg for hair loss who also starts ritonavir-boosted antiretroviral therapy, monitoring for increased sexual adverse effects (erectile dysfunction, decreased libido, reported incidence ~3.4% in key trials) is reasonable.

CYP3A4 Inducers with Finasteride

Rifampin (rifampicin), carbamazepine, phenytoin, and St. John's Wort accelerate CYP3A4 activity. Because CYP3A4 is only a minor finasteride pathway, the clinical significance of induction is low but not zero. One published case series noted reduced finasteride serum levels in patients on long-term rifampin for tuberculosis, though prostate volume outcomes were not reported. Prescribers treating BPH in patients on rifampin therapy might consider checking symptom scores (AUA Symptom Index) at 3-month intervals.

Drug-Disease Interactions Worth Noting

Finasteride is not renally cleared to a clinically meaningful degree, so renal impairment does not require dose adjustment. Hepatic impairment is a different story. The FDA label states that finasteride has not been studied in patients with hepatic insufficiency; because it is hepatically metabolized, levels may rise in Child-Pugh B or C disease. No specific dose recommendation exists, but caution is warranted.

Mechanism-Based Drug Interactions: Dutasteride

Dutasteride relies heavily on CYP3A4 and CYP3A5. This is not a minor metabolic pathway. The FDA label explicitly states that potent CYP3A4 inhibitors are expected to increase dutasteride exposure, though it stops short of mandating a dose adjustment given the drug's broad therapeutic index for BPH endpoints. The Avodart (dutasteride) prescribing information categorizes this as a potential interaction requiring clinical awareness.

Quantified CYP3A4 Interactions with Dutasteride

The most relevant data come from the dutasteride-verapamil interaction. A study found that verapamil (a moderate CYP3A4 inhibitor) increased dutasteride Cmax by 22% and AUC by 37%. Ketoconazole, a strong CYP3A4 inhibitor, increased dutasteride AUC by approximately 48 to 70% in pharmacokinetic modeling studies. These are not trivial changes in a drug with a 5-week half-life. A 48% AUC increase translates to prolonged supratherapeutic exposure that could persist for 6 to 8 weeks after stopping the inhibitor. Population pharmacokinetic analyses filed with the FDA confirm CYP3A4 as the rate-limiting clearance pathway.

Full DDI Table: Dutasteride

| Interacting Drug / Class | Interaction Type | Magnitude | Clinical Action | |---|---|---|---| | Ketoconazole, itraconazole | CYP3A4 strong inhibition | AUC +48 to 70% (estimated) | Monitor for adverse effects; consider temporary dose hold if azole course is short | | Verapamil, diltiazem | CYP3A4 moderate inhibition | AUC +37%; Cmax +22% | Monitor; no routine dose adjustment required | | Ritonavir, cobicistat | CYP3A4/5 strong inhibition | AUC increase likely >50% | Clinical monitoring; discuss benefit-risk with patient | | Clarithromycin | CYP3A4 strong inhibition | Moderate AUC increase | Use azithromycin for short-course infections where appropriate | | Rifampin, rifabutin | CYP3A4 strong induction | AUC decrease possible | May reduce BPH efficacy; monitor AUA symptom score | | Carbamazepine, phenytoin | CYP3A4 moderate induction | AUC decrease possible | Monitor symptom response | | St. John's Wort | CYP3A4 induction | AUC decrease | Advise patients to avoid unsupervised herbal supplementation | | Tamsulosin (combination therapy) | PK: minimal; PD: additive hypotension | Minor orthostatic risk | The CombAT trial used this combination; counsel on positional hypotension | | Amlodipine | CYP3A4 minor inhibition | Negligible | No action needed | | Warfarin | No known CYP2C9 interaction | None documented | Standard INR monitoring; no specific precaution | | Digoxin | Not a P-gp substrate interaction | None documented | No action needed | | Testosterone / exogenous androgens | Pharmacodynamic opposition | Reduces DHT from exogenous T | Monitor DHT levels if combination used for hypogonadism management |

Dutasteride and Testosterone Replacement Therapy

This combination is occasionally used off-label to suppress DHT in men on testosterone replacement therapy (TRT) who experience androgenetic alopecia or prostate enlargement. The interaction is pharmacodynamic, not pharmacokinetic. Exogenous testosterone increases substrate availability for 5-alpha reductase, and dutasteride blocks that conversion. In practice, serum DHT falls to near-castrate levels while testosterone remains in the therapeutic range. A randomized crossover study (N=40) published in the Journal of Clinical Endocrinology and Metabolism confirmed that dutasteride 0.5 mg added to testosterone therapy reduced serum DHT by 94% compared to testosterone alone.

Full Comparative DDI Table: Finasteride vs. Dutasteride

| Drug / Class | Finasteride Effect | Dutasteride Effect | Notes | |---|---|---|---| | Strong CYP3A4 inhibitors (ketoconazole, ritonavir, clarithromycin) | AUC +76% (ketoconazole data) | AUC +48 to 70% (estimated) | Dutasteride more affected due to major CYP3A4 dependence | | Moderate CYP3A4 inhibitors (verapamil, diltiazem, fluconazole) | Minimal | AUC +37% | Monitor dutasteride patients on calcium channel blockers | | CYP3A4 inducers (rifampin, carbamazepine, phenytoin) | Minor efficacy reduction | Potential efficacy reduction | Monitor BPH symptom scores | | St. John's Wort | Possible minor reduction | Possible minor reduction | Advise against | | Alpha-blockers (tamsulosin, terazosin) | Additive orthostatic hypotension (PD) | Additive orthostatic hypotension (PD) | The CombAT and MTOPS trials used combination therapy; counsel patients | | Testosterone / androgens (TRT) | DHT still produced (partial block) | DHT near-completely suppressed | Dutasteride preferred if maximal DHT suppression needed | | Warfarin | No interaction | No interaction | Standard monitoring | | NSAIDs / aspirin | No interaction | No interaction | No precaution | | 5-phosphodiesterase inhibitors (tadalafil, sildenafil) | No PK interaction; watch PD (combined sexual adverse effects) | No PK interaction; watch PD | Both classes linked to erectile dysfunction; combined use may increase subjective reporting | | SSRIs / SNRIs | No PK interaction | No PK interaction | Both drug classes associated with sexual dysfunction; combined use may increase adverse effect burden | | Finasteride + dutasteride | Do not combine | Do not combine | No additive clinical benefit; increased DHT suppression without proven outcome advantage |

Prescribing Guidance: BPH

The American Urological Association (AUA) 2023 guidelines on surgical and procedural interventions for BPH state that medical therapy with 5-ARIs is appropriate for patients with enlarged prostates (prostate volume >30 mL or PSA >1.5 ng/mL as a proxy). The AUA BPH guidelines note that symptom improvement takes 6 to 12 months to fully manifest. Prescribers should set that expectation upfront.

PSA Monitoring Protocol

Both agents reduce serum PSA by approximately 50% after 6 months of continuous use. This is not a sign of prostate cancer. The FDA label instructs clinicians to double the measured PSA value when interpreting results for men on 5-ARIs to approximate the true underlying value. Failure to account for this adjustment has caused missed prostate cancer diagnoses in clinical practice.

The PCPT trial (Prostate Cancer Prevention Trial, N=18,882) showed that finasteride reduced the incidence of low-grade prostate cancer by 24.8%, but initially raised concern about an increase in high-grade (Gleason 7 to 10) tumors. A 15-year follow-up analysis published in the New England Journal of Medicine found no statistically significant increase in prostate cancer mortality (P<0.05 for the original benefit on all-grade cancer), largely attributing the apparent high-grade excess to detection bias from smaller, more thoroughly sampled prostates. Prescribers should discuss this nuance during informed consent.

Dosing and Titration

Neither agent requires titration. Finasteride is dispensed at a fixed 5 mg daily for BPH or 1 mg daily for androgenetic alopecia. Dutasteride is 0.5 mg daily regardless of indication. No dose reduction is needed for renal impairment. Hepatic impairment requires caution but no defined dose adjustment.

Prescribing Guidance: Androgenetic Alopecia

Finasteride 1 mg (Propecia) is the only 5-ARI with FDA approval for androgenetic alopecia in men. The key registration trial enrolled 1,553 men aged 18 to 41 with mild-to-moderate vertex and anterior mid-scalp hair loss. At 2 years, 83% of finasteride-treated men had no further hair loss versus 28% of placebo patients; 66% showed visible regrowth on standardized photography. The drug must be used continuously. Hair gained during treatment is lost within 12 months of stopping.

Sexual Adverse Effects and Post-Finasteride Syndrome

Sexual adverse effects (erectile dysfunction, decreased libido, ejaculatory disorder) were reported in approximately 3.4 to 3.8% of finasteride 1 mg patients in key trials, versus 2.1% of placebo patients. Some patients report persistence of sexual adverse effects after discontinuation, a poorly understood phenomenon sometimes labeled post-finasteride syndrome. A 2020 systematic review in the Journal of Sexual Medicine found insufficient high-quality evidence to confirm a distinct syndrome but acknowledged persistent symptoms in a subset of patients. Prescribers should document the informed consent discussion about this possibility.

Finasteride in Women

Finasteride 5 mg daily has been studied off-label in postmenopausal women with androgenetic alopecia. A randomized trial (N=137) published in the Journal of the American Academy of Dermatology showed no significant difference between finasteride 1 mg and placebo in postmenopausal women, though higher doses (2.5 to 5 mg) show some benefit in premenopausal women who are reliably contracepting. Finasteride remains absolutely contraindicated in pregnant women and women of childbearing potential who are not using effective contraception. Even handling crushed tablets poses a teratogenic risk to male fetuses.

Special Populations

Elderly Patients

The half-life of finasteride extends to approximately 8 hours in men over 70 years old, compared to 6 hours in younger men. This difference is unlikely to be clinically significant for most patients. BPH is most prevalent in older men, and the AUA guidelines do not restrict use based on age alone.

Renal Impairment

Finasteride pharmacokinetics in patients with chronic renal impairment (creatinine clearance 9 to 55 mL/min) are not significantly different from patients with normal renal function. No dose adjustment is needed. Dutasteride is not renally cleared at meaningful levels; the same conclusion applies.

Hepatic Impairment

Finasteride has not been studied in patients with hepatic insufficiency. Because it is hepatically metabolized, exposure could increase in severe hepatic impairment. The same caution applies to dutasteride. Both labels advise clinical judgment; neither gives quantified guidance. Avoid in Child-Pugh C when alternatives exist.

Pediatric Patients

Neither agent is approved in pediatric patients. Finasteride is approved only in adult men. Dutasteride similarly carries no pediatric indication.

Monitoring Parameters for Prescribers

A structured monitoring approach reduces the risk of missed adverse effects and interaction-related complications.

At baseline:

• PSA (record before starting; this is the baseline to double-adjust for future readings)
• Digital rectal examination or prostate ultrasound to confirm enlarged prostate in BPH candidates
• Sexual function history (IIEF-5 score or equivalent)
• Medication reconciliation with specific attention to CYP3A4 inhibitors

At 6 months:

• Repeat PSA (expect approximately 50% reduction; if PSA has not fallen, consider non-adherence or underlying prostate pathology)
• AUA Symptom Index score for BPH patients
• Sexual function inquiry

Annually:

• Repeat PSA (adjusted)
• Review of current medication list for newly added CYP3A4 inhibitors or inducers
• Discuss continued benefit vs. Adverse effect burden

The Endocrine Society's clinical practice guideline on androgen therapy notes: "Regular monitoring of PSA is essential in men receiving androgen-modifying therapy, and clinicians should be familiar with the PSA adjustment required for 5-ARI use." Endocrine Society guidelines provide the broader hormonal context for these interactions.

Combination Therapy: 5-ARIs with Alpha-Blockers

The CombAT trial (Combination of Avodart and Tamsulosin, N=4,844) compared dutasteride 0.5 mg plus tamsulosin 0.4 mg against either agent alone over 4 years. Published in the European Urology journal and indexed on PubMed, CombAT demonstrated that combination therapy reduced the risk of acute urinary retention by 67.8% and BPH-related surgery by 70.6% compared with tamsulosin monotherapy. The combination did not introduce new pharmacokinetic interactions. The relevant concern is pharmacodynamic: additive alpha-blockade from tamsulosin and the vasodilatory effects of dutasteride's indirect androgen modulation can combine to cause orthostatic hypotension, particularly in men over 65.

The MTOPS trial (Medical Therapy of Prostatic Symptoms, N=3,047) similarly used finasteride 5 mg plus doxazosin 8 mg, finding that combination therapy reduced BPH clinical progression by 66% versus placebo, compared to 39% for finasteride alone and 39% for doxazosin alone. MTOPS results are available on PubMed. Prescribers combining these agents should start alpha-blockers at their lowest dose and titrate slowly.