Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
PROPECIA vs CIALIS
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Finasteride is a competitive and specific inhibitor of type II 5α-reductase, an intracellular enzyme that converts testosterone to dihydrotestosterone (DHT). By inhibiting 5α-reductase, finasteride reduces serum and intraprostatic DHT levels, decreasing androgenic stimulation of the prostate. In hair follicles, reduction of DHT levels slows hair loss and promotes hair regrowth.
Phosphodiesterase-5 (PDE5) inhibitor; increases c GMP levels, causing smooth muscle relaxation and vasodilation in the corpus cavernosum, enhancing erectile function.
Treatment of male pattern hair loss (androgenetic alopecia) in men only,Treatment of symptomatic benign prostatic hyperplasia (BPH) in men with an enlarged prostate
Treatment of erectile dysfunction,Treatment of benign prostatic hyperplasia,Treatment of pulmonary arterial hypertension (as Adcirca)
1 mg orally once daily
Tadalafil 10 mg or 20 mg orally as needed at least 30 minutes before sexual activity; maximum dosing frequency once daily. Alternative: 2.5 mg or 5 mg once daily for daily use.
Terminal elimination half-life is approximately 6-8 hours in young adults (range 4-12 hours), with clinical relevance for once-daily dosing; slightly prolonged in elderly (8-11 hours).
The terminal elimination half-life of tadalafil is approximately 17.5 hours in healthy subjects, which supports once-daily dosing for erectile dysfunction and once-daily use for benign prostatic hyperplasia. This long half-life distinguishes it from other PDE5 inhibitors.
Finasteride is extensively metabolized in the liver, primarily via the cytochrome P450 3A4 enzyme system. Two major metabolites, t-butyl side chain hydroxylation and ω-hydroxylation, have been identified; these metabolites possess less than 20% of the 5α-reductase inhibitory activity of finasteride.
Primarily hepatic via CYP3A4; minor pathways include CYP2C9 and glucuronidation.
Primarily hepatic metabolism; 57% excreted in feces (as metabolites), 39% in urine (as metabolites, <0.1% as unchanged finasteride).
Following oral administration, tadalafil is predominantly eliminated by hepatic metabolism. The metabolites are excreted mainly in feces (approximately 61% of the dose) and to a lesser extent in urine (approximately 36% of the dose). No unchanged parent drug is detected in urine.
Approximately 93% bound to plasma proteins (mainly albumin).
Tadalafil is 94% bound to plasma proteins, primarily to albumin. The protein binding is independent of drug concentration over a wide range.
Approximately 1.1 L/kg (range 0.9-1.3 L/kg), indicating extensive tissue distribution with penetration into seminal fluid and scalp tissue.
The apparent volume of distribution (Vd/F) is approximately 63 L (or roughly 0.9 L/kg for a 70 kg individual), indicating distribution into tissues beyond the vascular space, including the penis and other target organs.
Oral bioavailability is approximately 65% (range 60-70%); not affected by food.
Absolute oral bioavailability of tadalafil has not been formally determined; however, the drug is well absorbed after oral administration, with peak plasma concentrations reached in 0.5 to 6 hours (median 2 hours). Food does not affect the extent of absorption (AUC), though it may delay the rate (Tmax) by about 1–2 hours.
No dose adjustment required for any degree of renal impairment
Cr Cl 30-50 m L/min: 5 mg once daily (max) for daily use; as-needed dosing: 10 mg not to exceed once every 48 hours. Cr Cl <30 m L/min: not recommended. Hemodialysis: not studied.
No dose adjustment recommended; no studies in hepatic impairment
Child-Pugh A and B: no dose adjustment necessary for as-needed dosing; daily use: caution, start at 5 mg once daily. Child-Pugh C: not recommended.
Not indicated in pediatric patients; safety and efficacy not established
Not indicated for pediatric patients under 18 years.
No specific dose adjustment; limited data in elderly men with benign prostatic hyperplasia
No dose adjustment required solely based on age; consider renal function and concomitant medications.
PROPECIA is not approved for use in women or children. Finasteride is contraindicated in women who are or may become pregnant due to risk of abnormalities of the external genitalia of a male fetus. Women should not handle crushed or broken tablets when pregnant or may be pregnant.
None
Risk of prostate cancer: Finasteride may increase the risk of high-grade prostate cancer; digital rectal exam and PSA screening recommended before and during therapy.,Sexual dysfunction: Decreased libido, erectile dysfunction, ejaculation disorders, and decreased ejaculate volume have been reported; may persist after discontinuation.,Depression and suicidal ideation: Monitor for mood changes.,Breast cancer: Reported in men; evaluate any breast changes promptly.,Elevated PSA levels: Use caution interpreting PSA values in men on finasteride; adjust PSA levels by approximately 50% for clinical interpretation.,Hepatic impairment: Use with caution in patients with liver function abnormalities.,Pediatric use: Not indicated for use in children.
Risk of hypotension with vasodilators or alpha-blockers,Contraindicated with nitrates due to severe hypotension risk,Patients with left ventricular outflow obstruction (e.g., aortic stenosis) should avoid use,Caution in patients with hypotension, severe hepatic impairment, or end-stage renal disease,Risk of priapism: advise immediate medical attention for erections lasting >4 hours,Decreased visual or hearing ability requiring discontinuation
Hypersensitivity to finasteride or any component of the formulation,Women who are or may become pregnant (due to risk of hypospadias in male fetuses),Children (not indicated for use in pediatric patients)
Concomitant use of nitrates (any form) or riociguat,Hypersensitivity to tadalafil,Concomitant use with alpha-blockers (except for BPH with appropriate dosing)
No clinically significant food interactions. May be taken with or without food. However, avoid excessive alcohol intake as it may exacerbate certain side effects (e.g., dizziness).
Avoid high-fat meals prior to dosing as they may delay absorption and reduce peak plasma concentration. Avoid large quantities of grapefruit juice (more than 1 liter per day) as it may increase tadalafil exposure via CYP3A4 inhibition.
Contraindicated in females of childbearing potential. Finasteride inhibits conversion of testosterone to DHT, and risk of hypospadias in male fetuses if exposure occurs during gestation. No adequate studies in pregnant women; animal studies show abnormal external genitalia in male offspring at doses 1-100 times human exposure.
FDA Pregnancy Category B. Animal studies show no evidence of teratogenicity or embryotoxicity. No adequate, well-controlled studies in pregnant women. Risk cannot be ruled out; use only if clearly needed.
Not recommended. M/P ratio unknown. Finasteride is excreted in rat milk; no human data.
Excretion in human milk unknown. Not recommended for use in nursing mothers. M/P ratio not determined.
No dose adjustments applicable as drug is contraindicated in pregnancy.
No specific dose adjustments studied in pregnancy. Use lowest effective dose if necessary, with caution for increased plasma volume and renal clearance potentially altering pharmacokinetics.
Monitor patients for sexual dysfunction (e.g., decreased libido, erectile dysfunction) which may persist after discontinuation. Finasteride lowers serum PSA by approximately 50%; when interpreting PSA values in men taking Propecia, double the measured value for prostate cancer screening. Use with caution in patients with liver impairment; hepatic metabolism is primary clearance route. Avoid handling crushed or broken tablets in women who are or may become pregnant due to risk of teratogenicity (fetal genital abnormalities). Onset of hair regrowth typically takes 3-6 months; continue use for at least 12 months before assessing efficacy.
Tadalafil (Cialis) has a 17.5-hour half-life allowing once-daily dosing for ED or daily for BPH/LUTS. Avoid use with nitrates; may cause prolonged erection. Onset of action is 30-60 minutes, and effect may last up to 36 hours. Use with caution in patients with left ventricular outflow obstruction or severe hepatic impairment.
Take exactly as prescribed, usually one tablet (1 mg) daily with or without food.,Do not stop or skip doses without consulting your doctor; continuous use is needed to maintain benefit.,It may take 3-6 months to see hair regrowth and up to 12 months for full effect.,Report any new or worsening sexual side effects (e.g., decreased libido, erectile dysfunction, ejaculation disorders) promptly.,Finasteride may increase the risk of high-grade prostate cancer; discuss screening risks with your doctor.,Do not donate blood while taking Propecia and for at least 1 month after stopping to prevent exposure to pregnant women.,Women who are pregnant or may become pregnant should not handle crushed or broken tablets due to risk of birth defects.,If a dose is missed, skip it and take the next dose at the usual time; do not double up.
Do not take tadalafil if you take any form of nitrate medication (e.g., nitroglycerin) for chest pain.,Seek immediate medical help if you have an erection lasting more than 4 hours.,Avoid alcohol consumption as it may increase the risk of dizziness and low blood pressure.,Take tadalafil at least 30 minutes before sexual activity; effect can last up to 36 hours.,For daily use, take at the same time each day without regard to timing of sexual activity.,Grapefruit and grapefruit juice may increase tadalafil levels; avoid large amounts.,Inform your doctor of all medications you take, especially alpha-blockers, antihypertensives, and antifungal or antibiotic drugs.
No interactions on record
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about PROPECIA vs CIALIS, answered by our medical review team.
PROPECIA is a 5-alpha reductase inhibitor that works by Finasteride is a competitive and specific inhibitor of type II 5α-reductase, an intracellular enzyme that converts testosterone to dihydrotestosterone (DHT). By inhibiting 5α-reductase, finasteride reduces serum and intraprostatic DHT levels, decreasing androgenic stimulation of the prostate. In hair follicles, reduction of DHT levels slows hair loss and promotes hair regrowth.. CIALIS is a PDE5 Inhibitor that works by Phosphodiesterase-5 (PDE5) inhibitor; increases c GMP levels, causing smooth muscle relaxation and vasodilation in the corpus cavernosum, enhancing erectile function.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between PROPECIA and CIALIS depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of PROPECIA is: 1 mg orally once daily. The standard adult dose of CIALIS is: Tadalafil 10 mg or 20 mg orally as needed at least 30 minutes before sexual activity; maximum dosing frequency once daily. Alternative: 2.5 mg or 5 mg once daily for daily use.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between PROPECIA and CIALIS in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. PROPECIA is classified as Category C. Contraindicated in females of childbearing potential. Finasteride inhibits conversion of testosterone to DHT, and risk of hypospadias in male fetuses if exposure occurs during gest. CIALIS is classified as Category C. FDA Pregnancy Category B. Animal studies show no evidence of teratogenicity or embryotoxicity. No adequate, well-controlled studies in pregnant women. Risk cannot be ruled out; use. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.