Cialis and Finasteride Interaction: What You Need to Know

Are Tadalafil and Finasteride Safe to Take Together?

Tadalafil and finasteride can be taken together safely. No pharmacokinetic drug-drug interaction (DDI) has been identified between them in the FDA labeling for either agent, and no dose adjustment is required for either drug when they are co-prescribed. Clinicians frequently combine the two agents in men who have both benign prostatic hyperplasia (BPH) and erectile dysfunction (ED), since the conditions commonly coexist in the same patient population.

Why the Pair Is Commonly Prescribed Together

BPH and ED share several pathophysiological drivers: smooth-muscle tone in the prostate and penile vasculature, sympathetic adrenergic activity, and age-related androgen changes. A man presenting with lower urinary tract symptoms (LUTS) from BPH has roughly a 72% prevalence of concurrent ED, according to the Multinational Survey of the Aging Male (MSAM-7, N=12,815) [1]. Prescribing finasteride for prostate volume reduction alongside tadalafil for ED therefore addresses both conditions without requiring the patient to manage a dangerous interaction.

What the FDA Labels Say

The FDA prescribing information for tadalafil (Adcirca/Cialis) lists specific contraindicated combinations: organic nitrates (absolute), and cautionary guidance on alpha-blockers, antihypertensives, and CYP3A4 strong inhibitors such as ritonavir and ketoconazole [2]. Finasteride appears nowhere in tadalafil's contraindication or precaution list. Likewise, the FDA label for finasteride 5 mg (Proscar) identifies no interactions with PDE5 inhibitors [3]. Both documents were reviewed against the current 2024 label versions.

Pharmacokinetic Mechanism: How Each Drug Is Processed

Understanding why no significant interaction exists requires a brief look at how each drug moves through the body.

Tadalafil Metabolism

Tadalafil is metabolized predominantly by hepatic CYP3A4 into an inactive catechol metabolite. Its oral bioavailability is approximately 15%, and its plasma half-life is 17.5 hours, which is why once-daily dosing at 2.5 mg or 5 mg is feasible [2]. It is not a meaningful inhibitor or inducer of CYP3A4, CYP2C9, or P-glycoprotein at therapeutic concentrations.

A clinically significant pharmacokinetic interaction with a co-administered CYP3A4 substrate or inhibitor requires one drug to either substantially compete for the enzyme's active site or to up- or down-regulate the enzyme's expression. Tadalafil does neither.

Finasteride Metabolism

Finasteride is also metabolized by CYP3A4, producing two major inactive metabolites. Its oral bioavailability is approximately 63%, and the half-life is 5 to 6 hours (roughly 8 hours in men over 70) [3]. At therapeutic doses of 1 mg (for androgenetic alopecia) or 5 mg (for BPH), finasteride does not inhibit or induce CYP3A4 at concentrations achieved in plasma. It also shows no significant interaction with P-glycoprotein.

Why Two CYP3A4 Substrates Do Not Automatically Interact

Two drugs sharing the same metabolizing enzyme are commonly assumed to interact, but this is only clinically meaningful when one drug is a potent inhibitor or inducer of that enzyme. Neither tadalafil nor finasteride inhibits CYP3A4 at therapeutic plasma concentrations. A 2003 pharmacokinetic study evaluating tadalafil's interaction potential confirmed no clinically meaningful CYP inhibition at doses up to 20 mg [4]. Finasteride's own label specifies it has not been shown to affect the CYP enzyme system in a clinically relevant way [3].

Pharmacodynamic Considerations: Do They Affect the Same Pathways?

Pharmacodynamic interactions occur when two drugs act on overlapping physiological targets, producing additive, synergistic, or antagonistic effects. The tadalafil-finasteride pair has a nuanced pharmacodynamic relationship worth examining carefully.

Different Molecular Targets

Tadalafil inhibits phosphodiesterase type 5 (PDE5), preventing the breakdown of cyclic guanosine monophosphate (cGMP) in smooth-muscle cells. The resulting cGMP accumulation promotes smooth-muscle relaxation in the corpus cavernosum and prostatic stroma. This is tadalafil's mechanism for both ED relief and LUTS improvement.

Finasteride inhibits type II (and, to a lesser extent, type I) 5-alpha reductase (5-AR), the enzyme that converts testosterone to the more potent androgen dihydrotestosterone (DHT). Reducing intraprostatic DHT causes the prostate gland to shrink over 6 to 12 months, improving urinary flow.

These are entirely different molecular targets: PDE5 versus 5-AR. There is no direct receptor competition or shared second-messenger system.

Complementary Benefit in BPH-Related LUTS

One area where the pharmacodynamics interact productively is LUTS from BPH. Tadalafil 5 mg daily reduces the International Prostate Symptom Score (IPSS) by approximately 3.8 points at 12 weeks versus placebo, as shown in a pooled analysis of four randomized trials (N=1,058) [5]. Finasteride reduces IPSS by roughly 3.3 points and reduces prostate volume by 18% at 2 years in the Proscar Long-term Efficacy and Safety Study (PLESS, N=3,040) [6]. Their mechanisms are additive in reducing LUTS: tadalafil relaxes smooth muscle acutely, and finasteride shrinks the gland over months.

Androgen-Pathway Overlap and ED Risk

This is the one pharmacodynamic overlap that deserves careful attention. Both finasteride and sexual dysfunction share the androgen pathway. Finasteride suppresses DHT, which is not required for erection per se (erection is predominantly nitric-oxide-mediated), but DHT does contribute to libido, ejaculatory function, and neuropsychological aspects of sexual response.

The PLESS trial (N=3,040, 4 years) reported ED in 8.1% of finasteride 5 mg users versus 3.7% of placebo users, and decreased libido in 6.4% versus 3.4% [6]. For finasteride 1 mg (Propecia, used in hair loss), the rate of sexual adverse effects is lower but not zero. The FDA added a post-marketing label update in 2012 noting that libido decrease, ejaculatory disorders, and ED may persist after stopping the drug in a subset of patients [3].

Tadalafil, by directly improving erectile function through PDE5 inhibition, may partly offset the ED risk associated with finasteride. This is not merely theoretical. A 12-week randomized controlled trial (N=60) published in the International Journal of Impotence Research found that adding tadalafil 5 mg daily to finasteride 5 mg improved IIEF-EF (International Index of Erectile Function, Erectile Function domain) scores by a mean of 6.4 points versus finasteride alone [7]. Co-prescription of a PDE5 inhibitor with finasteride may therefore mitigate one of finasteride's most common reasons for patient discontinuation.

BPH Combination Therapy: The Clinical Evidence Base

The strongest clinical data for combination therapy in BPH comes from the Medical Therapy of Prostatic Symptoms (MTOPS) trial and related work. While MTOPS specifically examined doxazosin (an alpha-blocker) plus finasteride rather than tadalafil plus finasteride, it established the principle that a finasteride-based regimen benefits from a fast-acting co-agent that addresses symptoms while finasteride's volume reduction builds over months.

MTOPS Trial Findings

MTOPS (N=3,047, mean follow-up 4.5 years) compared placebo, doxazosin 8 mg, finasteride 5 mg, and their combination. The combination arm reduced the risk of overall clinical progression by 67% (hazard ratio 0.33, 95% CI 0.24-0.44, P<0.001) versus placebo, compared to 39% for doxazosin alone and 34% for finasteride alone [8]. The American Urological Association (AUA) 2021 BPH Guideline states: "Combination therapy with an alpha-blocker and a 5-alpha reductase inhibitor is recommended for patients with LUTS/BPH and prostate enlargement who desire treatment and have symptoms that are not adequately controlled by either monotherapy." [9]

Where Tadalafil Fits in BPH Guidelines

The AUA 2021 guideline also states that PDE5 inhibitors are an appropriate option for men with LUTS/BPH who also have ED, citing tadalafil 5 mg as FDA-approved for both conditions [9]. Tadalafil's FDA approval for BPH-associated LUTS was granted in 2011, making it the first PDE5 inhibitor with that indication [2]. Combining tadalafil 5 mg daily with finasteride 5 mg daily is therefore a well-supported clinical strategy, even if no single large RCT has been powered specifically on that exact combination.

CombAT and Related Data

The Combination of Avodart and Tamsulosin (CombAT) trial (N=4,844, 4 years) used dutasteride (a dual 5-AR inhibitor) rather than finasteride, but the principle is analogous. CombAT found the combination of a 5-ARI with an alpha-blocker produced greater IPSS improvement than either monotherapy at all time points [10]. Extrapolating to finasteride plus tadalafil is reasonable given overlapping mechanisms, though clinicians should note the data are most strong for 5-ARI plus alpha-blocker combinations.

Hypotension Risk: Setting the Record Straight

A common patient concern is whether tadalafil causes low blood pressure when combined with any other BPH drug. The answer depends entirely on which BPH drug is involved.

Alpha-Blockers: The Real Hypotension Warning

The FDA tadalafil label carries an explicit warning about additive hypotension when tadalafil is combined with alpha-blockers such as tamsulosin, doxazosin, and terazosin [2]. This occurs because both drug classes promote vascular smooth-muscle relaxation through different but convergent mechanisms. Men switching from a stable alpha-blocker to tadalafil (or adding tadalafil to an alpha-blocker) should begin with the lowest available tadalafil dose and be counseled on positional blood pressure changes.

Finasteride: No Meaningful Hemodynamic Overlap

Finasteride does not relax vascular smooth muscle. Its mechanism, 5-AR inhibition, affects androgen synthesis and has no direct hemodynamic effect. Adding finasteride to tadalafil does not augment tadalafil's mild blood-pressure-lowering effect. The mean reduction in systolic blood pressure with tadalafil 5 mg versus placebo is approximately 1 to 3 mmHg in normotensive men, and finasteride does not add to this [2]. Patients combining only tadalafil and finasteride (without an alpha-blocker or nitrate) do not require specific blood-pressure monitoring beyond routine clinical care.

Dosing Guidance for Combined Use

The following framework reflects current FDA labeling and AUA guideline recommendations for men using tadalafil and finasteride together.

Tadalafil Dosing in This Context

For ED: start at 10 mg as needed or 2.5 mg once daily, titrating to 20 mg as needed or 5 mg once daily based on response and tolerability [2].

For BPH-associated LUTS (with or without ED): tadalafil 5 mg once daily at approximately the same time each day. Dose reduction to 2.5 mg daily is recommended for men with a creatinine clearance of 30 to 50 mL/min [2].

Finasteride Dosing in This Context

For BPH: finasteride 5 mg once daily. Prostate volume reduction begins within 3 months and reaches its maximum effect at approximately 6 to 12 months [3]. Patients should be counseled that urinary symptom improvement from finasteride is slower than from tadalafil or alpha-blockers.

For androgenetic alopecia: finasteride 1 mg once daily. Men using this lower dose for hair loss who also have ED may still benefit from tadalafil; the combination is safe at both dose levels.

No Dose Adjustment Required

Neither the tadalafil nor the finasteride dose requires adjustment when they are co-administered. There is no clinically meaningful pharmacokinetic interaction, so no compensatory dose change is warranted [2, 3].

Monitoring and Patient Counseling

What to Monitor

Baseline and periodic tracking should include:

• International Index of Erectile Function (IIEF-EF) score to document sexual function before and after starting finasteride, so any decline is detectable and attributable.
• IPSS score at baseline, 3 months, and 12 months to assess LUTS trajectory.
• PSA (prostate-specific antigen) at baseline and 6 months. Finasteride typically reduces PSA by approximately 50% after 6 to 12 months; failure to see this reduction may signal non-compliance or prostate cancer [3]. Laboratories should double the measured PSA value to estimate the true unfinasteride-adjusted level.
• Blood pressure at routine visits, particularly if an alpha-blocker is added later.

Counseling Points for Patients

Patients deserve straightforward, specific information. Four points are non-negotiable:

1. Finasteride takes 6 to 12 months for full prostate volume benefit. Tadalafil works within 30 minutes to 2 hours for ED and provides LUTS benefit within weeks.
2. Sexual side effects from finasteride (ED, decreased libido, ejaculatory changes) occur in a meaningful minority of patients. If they appear, tadalafil may help with ED specifically, but libido changes may require separate discussion.
3. Finasteride's sexual side effects can persist after stopping the drug in rare cases. Men should report persistent symptoms rather than assuming they will resolve spontaneously.
4. Tadalafil must never be combined with nitrates (nitroglycerin, isosorbide mononitrate, amyl nitrite poppers) regardless of whether finasteride is in the regimen. This contraindication is absolute and unrelated to finasteride.

Special Populations

Renal Impairment

Tadalafil requires dose adjustment in moderate-to-severe renal impairment (creatinine clearance <30 mL/min), where the maximum recommended dose is 5 mg once daily or 10 mg as needed no more than once every 48 hours [2]. Finasteride does not require renal dose adjustment because its metabolites are excreted primarily in feces rather than urine [3].

Hepatic Impairment

Tadalafil should not exceed 10 mg as needed in mild-to-moderate hepatic impairment (Child-Pugh Class A or B) and is not recommended in severe impairment (Class C) [2]. Finasteride is also primarily hepatically metabolized; use with caution in significant hepatic disease [3].

Older Men

Men over 65 may have slower CYP3A4 activity, but the tadalafil label does not mandate dose reduction based on age alone [2]. Finasteride half-life extends to approximately 8 hours in men over 70, though this has not required dose adjustment in clinical trials [3]. Older men are more likely to be on alpha-blockers concurrently, which does require careful tadalafil titration.