Enclomiphene Citrate and Tadalafil Interaction

Why These Two Drugs Are Frequently Co-Prescribed

Men diagnosed with secondary hypogonadism often present with two overlapping complaints: low testosterone and erectile dysfunction. Enclomiphene citrate, the trans-isomer of clomiphene, raises endogenous testosterone by blocking estrogen negative feedback at the hypothalamus and pituitary [1]. Tadalafil, a phosphodiesterase type 5 (PDE5) inhibitor, treats ED by increasing cyclic GMP-mediated smooth muscle relaxation in the corpus cavernosum [2].

The Clinical Overlap

Because low testosterone itself contributes to ED, many clinicians prescribe enclomiphene to address the hormonal root cause while adding tadalafil for symptomatic relief during the weeks it takes testosterone levels to normalize. A 2019 retrospective chart review of 400 men treated with clomiphene citrate for hypogonadism found that 38% were concurrently prescribed a PDE5 inhibitor [3]. The combination targets two distinct physiological deficits simultaneously.

Why Patients Ask About This Interaction

The question is reasonable. Both drugs affect male reproductive and sexual physiology. Patients worry about overlapping side effects, hormonal disruption, or cardiovascular strain. The reassuring finding is that these drugs operate through unrelated pathways with minimal metabolic competition.

Pharmacokinetic Profile: Separate Metabolic Highways

A meaningful drug-drug interaction typically requires shared metabolic enzymes, transporter competition, or protein-binding displacement. Enclomiphene and tadalafil share none of these.

Enclomiphene Metabolism

Enclomiphene citrate is hepatically metabolized, primarily through CYP2D6 with minor contributions from CYP3A4 [1]. Its half-life is approximately 10 hours. It does not meaningfully inhibit or induce CYP3A4 at therapeutic doses (12.5 to 25 mg daily).

Tadalafil Metabolism

Tadalafil is metabolized predominantly by CYP3A4, with the resulting catechol metabolite undergoing further glucuronidation [2]. Its half-life is 17.5 hours, the longest among PDE5 inhibitors. Strong CYP3A4 inhibitors (ketoconazole, ritonavir) increase tadalafil exposure by up to 312%, and strong CYP3A4 inducers (rifampin) reduce it by 88% [2]. Enclomiphene is neither a strong inhibitor nor an inducer of CYP3A4.

P-glycoprotein and Protein Binding

Tadalafil is approximately 94% protein-bound. Enclomiphene is also highly protein-bound. Displacement interactions become clinically significant only when a drug is both highly protein-bound (above 90%) and has a narrow therapeutic index. Tadalafil has a wide therapeutic window (doses from 2.5 to 20 mg are all approved), so minor shifts in free fraction are unlikely to produce toxicity [2].

No P-glycoprotein (P-gp) transporter interaction between these two agents has been reported.

Pharmacodynamic Analysis: Additive, Not Antagonistic

The pharmacodynamic question is whether the combined effect on vasculature, hormones, or cardiac conduction creates risk.

Hormonal Effects

Enclomiphene raises luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which stimulate Leydig cells to produce more testosterone. In ZA-301 and ZA-302 (Phase III trials, N=267 combined), enclomiphene 12.5 mg and 25 mg daily raised total testosterone into the normal range (above 300 ng/dL) in 73% and 79% of men, respectively, by week 12 [4]. Tadalafil does not directly affect testosterone synthesis. A 2018 meta-analysis of 17 RCTs (N=1,101) found no statistically significant change in serum testosterone with PDE5 inhibitor use alone [5].

The combined hormonal effect is therefore driven almost entirely by enclomiphene. There is no opposing or compounding hormonal mechanism.

Cardiovascular Effects

Tadalafil produces mild systemic vasodilation. The FDA label reports mean reductions of 1.6 mmHg systolic and 0.8 mmHg diastolic at 20 mg [2]. This effect becomes dangerous only when combined with nitrates or alpha-blockers at non-titrated doses.

Enclomiphene has no established vasodilatory or vasoconstrictive properties. Clomiphene citrate (the racemic mixture) has been associated with rare thromboembolic events, attributed primarily to the zuclomiphene (cis) isomer, which accumulates due to a half-life exceeding 30 days [6]. Enclomiphene, the isolated trans-isomer, clears much faster (half-life ~10 hours) and does not accumulate to the same degree.

Venous Thromboembolism Consideration

The Endocrine Society's 2018 guidelines on male hypogonadism note that clomiphene citrate use carries a theoretical thrombotic risk, though population-level data are limited [7]. Tadalafil at 5 mg daily has been studied for pulmonary arterial hypertension (the PHIRST trial, N=405) and showed a vasodilatory, not prothrombotic, profile [8].

If a patient has independent VTE risk factors (obesity, factor V Leiden, immobility), the relevant drug to monitor is enclomiphene, not the combination.

DDI Database Ratings and Guideline Positions

What the Major Databases Say

Neither Lexicomp nor Micromedex lists an interaction between enclomiphene (or clomiphene) and tadalafil as of May 2026. The FDA-approved label for Cialis (tadalafil) lists interactions with nitrates, alpha-blockers, strong CYP3A4 inhibitors, and antihypertensives, but does not mention SERMs or clomiphene-class agents [2].

The Repros Therapeutics NDA submission for enclomiphene (Androxal) included drug interaction data showing no clinically significant interaction with CYP3A4-metabolized substrates [4]. The FDA did not approve Androxal (citing efficacy-endpoint disagreements), but the pharmacokinetic data in the review remain publicly available.

Endocrine Society Position

The Endocrine Society's 2018 Clinical Practice Guideline on testosterone therapy recommends clomiphene as an off-label alternative to exogenous testosterone for men who wish to preserve fertility [7]. The guideline does not address PDE5 inhibitor co-administration specifically, which itself reflects the low concern level: high-risk interactions are always called out in major practice guidelines.

Monitoring Recommendations for Co-Prescribed Patients

Even without a formal DDI, men taking both drugs deserve structured follow-up. Hypogonadism and ED are conditions that require ongoing laboratory and clinical assessment.

Baseline Labs Before Starting

Before starting enclomiphene, obtain: total testosterone (drawn between 7:00 and 10:00 AM on two separate mornings), free testosterone, LH, FSH, estradiol, complete metabolic panel including liver enzymes, complete blood count with hematocrit, and lipid panel [7]. Tadalafil requires assessment of cardiovascular fitness for sexual activity, blood pressure measurement, and a medication reconciliation specifically checking for nitrate or alpha-blocker use [2].

Follow-Up Schedule

Recheck testosterone, estradiol, LH, and FSH at 4 to 8 weeks after initiating enclomiphene. If testosterone normalizes (above 450 ng/dL in most clinical targets), some patients may experience sufficient erectile improvement to reduce or discontinue tadalafil. If estradiol rises disproportionately (above 50 pg/mL), clinicians may lower the enclomiphene dose from 25 mg to 12.5 mg daily or add a low-dose aromatase inhibitor [7].

Liver Function

Both drugs undergo hepatic metabolism. Check ALT and AST at baseline, at 3 months, and annually. The FDA label for clomiphene notes rare hepatotoxicity, and tadalafil is contraindicated in severe hepatic impairment (Child-Pugh C) [1][2].

Hematocrit

Unlike exogenous testosterone, enclomiphene does not typically cause polycythemia because it raises testosterone through endogenous production, which is self-regulated by the HPG axis. A hematocrit check at baseline and at 6 months is still prudent.

Dose Adjustments: When Are They Needed?

Standard Dosing

Enclomiphene is typically dosed at 12.5 mg or 25 mg by mouth once daily. Tadalafil for daily use is 2.5 mg or 5 mg; for on-demand use, 10 mg or 20 mg taken before sexual activity [2].

Scenarios Requiring Adjustment

No dose adjustment of either drug is needed based solely on co-administration. Adjustments become necessary in these clinical situations:

• CYP3A4 inhibitor added to the regimen. If a patient starts a moderate CYP3A4 inhibitor (erythromycin, fluconazole), tadalafil exposure may increase. The FDA recommends limiting tadalafil to 2.5 mg daily or 10 mg no more than once every 72 hours in this scenario [2]. Enclomiphene dose does not change.
• Hepatic impairment. In mild-to-moderate hepatic impairment (Child-Pugh A or B), tadalafil should not exceed 10 mg on-demand. Enclomiphene has not been formally studied in hepatic impairment, but conservative dosing (12.5 mg daily) is recommended.
• Renal impairment. For creatinine clearance 30 to 50 mL/min, no tadalafil adjustment is needed for the 5 mg daily dose. For clearance below 30 mL/min, the starting dose should be 2.5 mg daily [2].

Patient Counseling Points

What to Tell Patients

Patients should know three things. First, there is no known interaction between enclomiphene and tadalafil that requires avoiding the combination. Second, the absolute contraindication that matters is tadalafil plus nitrates (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate, amyl nitrite). This contraindication applies regardless of whether enclomiphene is in the picture [2]. Third, any visual changes (blurred vision, color tinge) should be reported immediately, as both clomiphene-class drugs and PDE5 inhibitors have rare ocular side effects [1][2].

When to Seek Urgent Care

Priapism (erection lasting over 4 hours) is a urologic emergency. The risk with tadalafil alone is very low (reported in fewer than 1 in 10,000 patients), and enclomiphene does not increase this risk [2]. Still, patients should be instructed to go to an emergency department if this occurs.

Sudden vision loss in one or both eyes (non-arteritic anterior ischemic optic neuropathy, NAION) has been reported rarely with all PDE5 inhibitors. The FDA label notes the association without establishing causation [9].

Special Populations

Younger Men Seeking Fertility Preservation

Enclomiphene is often chosen over exogenous testosterone specifically because it preserves spermatogenesis. In the ZA-302 trial, men on enclomiphene 25 mg maintained sperm concentrations above 15 million/mL (the WHO lower reference limit), while men on topical testosterone gel saw concentrations drop below 5 million/mL [4]. Adding tadalafil does not affect spermatogenesis.

Men Over 65

The tadalafil FDA label notes that healthy elderly subjects (65 and older) had 25% higher tadalafil exposure (AUC) compared to younger subjects, but this did not warrant a dose adjustment [2]. Enclomiphene pharmacokinetics in elderly men have not been formally characterized. Clinicians should start at 12.5 mg daily and titrate based on testosterone response.

Men on Antihypertensives

Tadalafil's mild vasodilatory effect is additive with antihypertensives. In a study of tadalafil 20 mg co-administered with amlodipine 5 mg, the mean additional blood pressure decrease was 3/3 mmHg beyond amlodipine alone [2]. This is usually tolerable but should prompt blood pressure checks at follow-up visits.