Can I Take Quercetin with Cialis (Tadalafil)?

What Is the Quercetin, Tadalafil Interaction?

Quercetin can inhibit CYP3A4, the liver enzyme responsible for metabolizing tadalafil. When CYP3A4 activity is reduced, tadalafil clearance slows and plasma concentrations rise above the expected range. Higher tadalafil levels increase the likelihood of vasodilatory side effects: hypotension, flushing, and headache in particular.

Tadalafil's Metabolic Pathway

Tadalafil is almost entirely cleared through hepatic CYP3A4-mediated oxidation. The FDA-approved label for Cialis states that co-administration with ketoconazole 200 mg daily, a potent CYP3A4 inhibitor, increased tadalafil AUC by 107% and peak concentration (Cmax) by 15% [1]. The label specifically recommends limiting tadalafil to 10 mg no more than once every 72 hours when any potent CYP3A4 inhibitor is present [1].

Quercetin is not as potent as ketoconazole, but it is not inert either.

How Quercetin Inhibits CYP3A4

Quercetin (3,3',4',5,7-pentahydroxyflavone) is a flavonoid found in onions, capers, and apples, and sold in concentrated supplement form at 250 to 1,000 mg per capsule. A 2005 study by Moon and colleagues published in Drug Metabolism and Disposition demonstrated that quercetin inhibits CYP3A4 activity in human liver microsomes with a Ki in the low-micromolar range [2]. Oral supplementation with 500 mg quercetin twice daily has been shown to produce plasma concentrations in the range of 0.5 to 1.5 micromolar in healthy adults [3], concentrations overlapping with those that produced measurable enzyme inhibition in vitro.

A 2016 pharmacokinetic study in European Journal of Drug Metabolism and Pharmacokinetics confirmed that quercetin co-administration altered the plasma pharmacokinetics of CYP3A4 substrate drugs in a clinically detectable way [4]. The magnitude of that shift depends on the quercetin dose, formulation bioavailability, and individual CYP3A4 expression, which varies up to 40-fold between people [5].

Is the Interaction Pharmacokinetic, Pharmacodynamic, or Both?

The primary concern is pharmacokinetic: quercetin slows how fast your body clears tadalafil. A secondary, pharmacodynamic concern also exists because both compounds independently lower blood pressure through different routes.

The Pharmacokinetic Component

Tadalafil has a mean half-life of 17.5 hours in healthy adults [1]. If CYP3A4 is partially inhibited by concurrent quercetin, that half-life effectively lengthens. A drug whose clearance is reduced by even 30 to 40% will accumulate to a higher steady-state concentration with repeated daily dosing. For men taking tadalafil 5 mg daily for BPH or low-dose ED maintenance, accumulation over several days may push plasma levels into ranges more typical of the 10 or 20 mg on-demand dose.

The Pharmacodynamic Component

Quercetin has measurable vasodilatory properties independent of CYP3A4. A 2007 randomized trial published in the Journal of Nutrition (N=93) found that 730 mg quercetin daily for eight weeks reduced systolic blood pressure by 7.0 mmHg in hypertensive adults [6]. Tadalafil itself reduces blood pressure by 1 to 3 mmHg at therapeutic doses, and that reduction is amplified substantially when combined with alpha-blockers or antihypertensives [1]. Adding quercetin's modest antihypertensive effect on top of tadalafil's creates additive downward pressure on blood pressure, even if tadalafil levels were not elevated at all.

P-Glycoprotein and Intestinal Absorption

A third, smaller mechanism deserves mention. Quercetin also inhibits intestinal P-glycoprotein, an efflux transporter that limits drug absorption at the gut wall [7]. If P-glycoprotein is suppressed, a greater fraction of an oral tadalafil dose may be absorbed before first-pass metabolism begins. This could modestly raise Cmax even before hepatic CYP3A4 inhibition takes effect.

What the Evidence Actually Shows

No large randomized human trial has examined the quercetin, tadalafil combination directly. The evidence base is built from in vitro enzyme assays, single-substrate pharmacokinetic studies, and by analogy from better-studied CYP3A4 inhibitor pairs involving other PDE5 inhibitors.

In Vitro and Animal Data

The Moon 2005 Drug Metabolism and Disposition paper [2] and a follow-up 2010 study in Xenobiotica both show that quercetin's inhibition of CYP3A4 is concentration-dependent and reversible, meaning it is a competitive rather than mechanism-based (irreversible) inhibitor [8]. Competitive inhibitors are generally less clinically serious than mechanism-based inhibitors like grapefruit furanocoumarins, but the practical difference depends on how long quercetin plasma levels stay elevated. At a 500 mg twice-daily dose, quercetin remains detectable in plasma for six to eight hours post-ingestion [3].

Analogy from Sildenafil Studies

Sildenafil shares the CYP3A4 metabolic pathway with tadalafil. A 2002 study in British Journal of Clinical Pharmacology showed that erythromycin, a moderate CYP3A4 inhibitor with roughly comparable inhibition potency to high-dose quercetin, raised sildenafil AUC by 182% and Cmax by 56% [9]. That finding supports the plausibility of a clinically meaningful rise in tadalafil exposure from quercetin co-administration, even though tadalafil's longer half-life and lower sensitivity to CYP3A4 fluctuations makes it somewhat less reactive than sildenafil to enzyme inhibition.

What the FDA Label Says

The Cialis prescribing information categorizes inhibitors into potent (ketoconazole, ritonavir), moderate (erythromycin, fluconazole), and no formal category for supplements. It does not list quercetin by name [1]. However, the label's general language specifies: "The use of tadalafil with drugs that are potent inhibitors of CYP3A4 may require dosage reduction." Quercetin at supplement doses occupies a moderate-inhibitor tier by the available kinetic data, not a potent tier. That distinction matters for risk estimation.

Who Is at the Greatest Risk?

Not everyone combining quercetin and tadalafil faces the same level of risk. Several patient-specific factors shift the probability of a clinically meaningful interaction.

Higher-Risk Profiles

Men taking daily-dose tadalafil 5 mg (rather than on-demand 10 or 20 mg) may accumulate the most risk because daily dosing creates a steady-state blood level that is already continuous. Any inhibition of clearance compounds on that baseline.

Men also taking alpha-blockers such as tamsulosin 0.4 mg or doxazosin 4 mg are at amplified hypotension risk. The Cialis label includes a boxed warning about this combination [1], and adding quercetin's antihypertensive effect to that already-cautioned pairing creates a three-way additive interaction on blood pressure.

Age matters as well. Adults over 60 have a mean 25% reduction in CYP3A4 hepatic clearance capacity compared to younger adults, according to data from the NIH pharmacogenomics literature [5], meaning they start from a lower metabolic baseline and have less reserve to absorb any additional enzyme inhibition.

Lower-Risk Profiles

A man taking on-demand tadalafil 10 mg once weekly with no other cardiovascular medications, normal blood pressure, and quercetin at a dietary dose (e.g., two to three servings of quercetin-rich vegetables per day, roughly 25 to 50 mg total quercetin), faces a far smaller risk. Dietary quercetin at those amounts is unlikely to reach plasma concentrations high enough to produce meaningful CYP3A4 inhibition.

The risk is primarily tied to supplement-grade doses: 500 mg or more per day in capsule or tablet form.

Practical Guidance: What to Do If You Take Both

The most important step is disclosing quercetin supplementation to whoever prescribes your tadalafil. This is true even if you are obtaining tadalafil through a telehealth platform. Supplement use is frequently omitted from medication histories, and no interaction check will catch it if it is not entered.

Dose-Separation Strategy

If you choose to continue both while waiting for prescriber review, separate the doses by at least four hours. Quercetin plasma levels peak within one to two hours of an oral dose and fall off substantially by four hours [3]. Taking tadalafil at least four hours after quercetin reduces but does not eliminate the overlap window during which enzyme inhibition is maximal. This separation strategy is a bridging measure, not a permanent solution.

Monitoring for Symptoms

Symptoms that may indicate elevated tadalafil exposure include: a blood pressure drop that causes dizziness when standing (orthostatic hypotension), flushing or severe headache lasting more than four hours, visual disturbances, or, in rare cases, a prolonged erection lasting more than four hours (priapism), which requires emergency care. If any of these occur, stop the supplement, contact your prescriber, and if symptoms are severe, seek same-day medical evaluation.

Dose Adjustment Considerations

Your prescriber may elect to reduce tadalafil dose rather than discontinue quercetin, particularly if you are using quercetin for a documented purpose such as adjunct management of chronic prostatitis (for which a 2009 randomized controlled trial published in Urology showed quercetin 500 mg twice daily reduced NIH-CPSI scores by a mean of 5.7 points vs. 1.4 for placebo, P<0.001, N=30) [10]. The combination of quercetin and tadalafil has theoretical appeal in that indication, since both agents may reduce pelvic inflammatory signaling, but dose management becomes even more relevant if both are used together.

Quercetin's Own Evidence Base for Relevant Conditions

Quercetin is not simply a passive bystander in this interaction. Men taking Cialis for ED or BPH are sometimes also interested in quercetin for its antioxidant, anti-inflammatory, or prostate-related properties. Understanding what the evidence supports helps put the interaction in context.

Erectile Dysfunction

Animal studies have shown that quercetin improves endothelial nitric oxide synthase (eNOS) activity in penile tissue, which mirrors one pathway through which PDE5 inhibitors work [11]. A 2011 study in the Journal of Sexual Medicine demonstrated that quercetin supplementation improved erectile function scores in a diabetic rat model, suggesting possible additive pharmacodynamic benefit alongside PDE5 inhibitor therapy [11]. No adequately powered human RCT has confirmed this effect in men with ED to date.

Benign Prostatic Hyperplasia

Quercetin inhibits 5-alpha reductase activity in prostate cell lines, according to data published in Biological and Pharmaceutical Bulletin in 2004 [12]. Some men prescribed tadalafil 5 mg for BPH may add quercetin expecting synergistic anti-proliferative effects. The 5-alpha reductase angle is separate from the CYP3A4 concern but adds to the overall complexity of managing both agents simultaneously.

Prostatitis

The 2009 Urology RCT by Shoskes and colleagues (N=30) remains the most cited human trial for quercetin in chronic prostatitis/chronic pelvic pain syndrome [10]. Since tadalafil is also used off-label for pelvic floor relaxation and prostatitis symptom relief in some men, the combination has clinical rationale. That rationale does not remove the need for careful pharmacokinetic management.

What Your Prescriber Will Assess

When you disclose quercetin use, a prescriber will typically run through a short checklist before deciding whether to adjust the tadalafil dose, change the quercetin dose, or simply note the combination and monitor.

Key questions include: What tadalafil dose and dosing frequency are you currently on? Are you also taking alpha-blockers, antihypertensives, or nitrates? What is your resting blood pressure? Have you noticed any unusual symptoms since starting quercetin? What is the quercetin dose and formulation (plain aglycone vs. Quercetin phytosome, which has significantly higher bioavailability)?

Quercetin phytosome formulations (e.g., Quercefit) can produce plasma levels two to three times higher than standard quercetin at equivalent doses [13], which shifts the inhibition risk upward. If your supplement is a phytosome or "bioavailable" formulation, make that clear.

Summary of Interaction Severity by Scenario

| Scenario | Estimated Interaction Severity | |---|---| | Dietary quercetin (25 to 50 mg/day from food) + tadalafil any dose | Minimal | | Quercetin 250 mg/day supplement + tadalafil 5 mg daily | Low to moderate | | Quercetin 500 to 1,000 mg/day + tadalafil 5 mg daily + alpha-blocker | Moderate to high | | Quercetin phytosome 500 mg/day + tadalafil 20 mg on-demand | Moderate | | Any quercetin dose + tadalafil + nitrate (e.g., nitroglycerin) | Contraindicated regardless of quercetin (due to nitrate-tadalafil absolute contraindication) |

The nitrate row applies independently of quercetin; tadalafil is absolutely contraindicated with any form of nitrate therapy per FDA labeling [1].