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Finasteride (Propecia / Proscar)

5-Alpha Reductase Inhibitorprescriptionoral · inferred

Selective type II 5-alpha reductase inhibitor that blocks conversion of testosterone to dihydrotestosterone (DHT). 1 mg for male pattern hair loss (Propecia), 5 mg for BPH (Proscar). Despite short plasma half-life, a single dose suppresses serum DHT for up to 4 days due to high enzyme affinity.

Projected serum levels — 1 mg, once daily

Finasteride (Propecia / Proscar)
Finasteride (Propecia / Proscar) modeled serum levels, 1 mg once daily over 15 days Population-based pharmacokinetic estimate. Steady state reached after approximately 1 days. 0 0.25 0.50 0.75 1 Day 0 Day 4 Day 8 Day 11 Day 15 Time on a regular schedule ≈ steady state · day 1

Maintenance schedule: 1 mg once daily (oral).

Modeled steady state after ~1 days: peak ≈ 0.73 mg, trough ≈ 0.056 mg body load. Population-based estimate over 15 days for a 1 mg dose at a 70 kg reference body mass — the interactive app scales curves to your doses, timing, and body mass.

Key facts

Category
5-Alpha Reductase Inhibitor
Route modeled
Oral
Model confidence
inferred
Half-life
6 h
Common dose
1 mg
Suggested maximum
5 mg/day
Reference dose range
0.50–5 mg (single dose)
Suggested cadence
once daily
Validated against
5 mg oral — Cmax 0.037 mg/L at 1.5 h

Documented interactions

  • contraindicated
    Carbamazepine (Tegretol) + Finasteride (Propecia / Proscar) CYP induction

    Carbamazepine (Tegretol) inducer of CYP3A4 predicted to change Finasteride (Propecia / Proscar) AUC by ~0.20x

  • contraindicated
    Clarithromycin (Biaxin) + Finasteride (Propecia / Proscar) CYP inhibition

    Clarithromycin (Biaxin) mechanism_based of CYP3A4 predicted to change Finasteride (Propecia / Proscar) AUC by ~10.00x

  • contraindicated
    Dasatinib (Sprycel) + Finasteride (Propecia / Proscar) CYP inhibition

    Dasatinib (Sprycel) time_dependent_inhibitor of CYP3A4 predicted to change Finasteride (Propecia / Proscar) AUC by ~5.00x

  • contraindicated
    Diltiazem + Finasteride (Propecia / Proscar) CYP inhibition

    Diltiazem mechanism_based of CYP3A4 predicted to change Finasteride (Propecia / Proscar) AUC by ~10.00x

  • contraindicated
    Efavirenz (Sustiva) + Finasteride (Propecia / Proscar) CYP induction

    Efavirenz (Sustiva) inducer of CYP3A4 predicted to change Finasteride (Propecia / Proscar) AUC by ~0.12x

  • contraindicated
    Grapefruit (whole fruit) + Finasteride (Propecia / Proscar) CYP inhibition

    Grapefruit (whole fruit) mechanism_based of CYP3A4 predicted to change Finasteride (Propecia / Proscar) AUC by ~10.00x

Serum checks 63 modeled interaction pairings for finasteride (propecia / proscar) across your whole stack — absorption conflicts, enzyme inhibition, and nutrient depletion included.

Research behind this entry

  1. Finasteride in the treatment of men with androgenetic alopecia Kaufman KD et al. · Journal of the American Academy of Dermatology, 1998 DOI

    Pivotal 2-year RCT showing finasteride 1 mg/day increased hair count by 107 hairs at 1 year and 138 hairs at 2 years versus placebo, establishing efficacy for male pattern hair loss.

  2. The effect of finasteride on the risk of acute urinary retention and the need for surgical treatment among men with BPH (PLESS) McConnell JD et al. · New England Journal of Medicine, 1998 DOI

    PLESS trial of 3,040 men with BPH showed finasteride 5 mg reduced risk of acute urinary retention by 57% and need for surgery by 55% over 4 years versus placebo.

  3. Clinical pharmacokinetics and pharmacodynamics of finasteride Steiner JF · Clinical Pharmacokinetics, 1996 DOI

    Definitive PK/PD review establishing finasteride half-life of 4.7-7.1h, oral bioavailability of 80%, and persistent DHT suppression lasting days despite short plasma half-life due to tight enzyme binding.

3 published studies referenced in the app, each with a plain-language summary.